Plasma nitriding

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In plasma nitriding and nitrocarburizing , nitrogen is specifically diffused into the edge zone of iron-based alloys in an ionized gas atmosphere . The plasma nitriding process is used to give functional surfaces a higher surface hardness so that they have increased resistance to abrasive , adhesive and corrosive wear .

history

Historical finds prove that nitration or nitrocarburization took place as early as before Christ (even if only naturally):

Carbonitrided workpieces from around 100 BC have been found in China. Iron columns / rods with nitrogen gradients (natural nitriding) on ​​the surface have existed in India since 415 AD. At the beginning of the 20th century, Adolph Machelet wrote the first work on gas nitriding in the USA (first patents 1908). This was followed by Adolph Fry in Germany with studies on the influence and role of nitrogen on alloying elements (patent from 1921). Salt bath nitriding gained more and more recognition in the 1920s after Hermann Schlosser from the USA brought his first experience with cyanide-containing molten salts for wear-resistant surfaces to Europe. Klöckner Ionon GmbH started with its first plasma nitriding systems in 1932. In 1944/1945 military applications found pleasure in it (nitriding of gun barrels, ...). Industrial acceptance increased in Europe in the 1970s. The warm wall technology was developed in the 1980s. Alternative methods such as “plasma immersion ion implantation” or “active grids” were developed in the 1990s. Today plasma nitriding technology is accepted worldwide.

functionality

The process works with a nitrogen- hydrogen gas mixture as the nitriding medium , which is ionized in a vacuum furnace at a negative pressure of 50 Pa to 600 Pa by means of a powerful glow discharge . Due to the high energetic effect of the plasma, the working temperature during plasma nitriding can be reduced considerably compared to salt bath and gas nitriding , so that this process can also be used for warp-sensitive materials and components. The typical working range for plasma nitriding iron alloys is between 350 ° C and 600 ° C. If the high voltage that is applied between the furnace wall and the charge exceeds a critical value after switching on , a glow discharge ignites. The plasma causes diffusible nitrogen to be formed on the surface , which can then diffuse up to 0.8 mm deep into the edge zone with certain steels if the component temperature is sufficiently high and the treatment time is correspondingly long. Depending on the process, a pure diffusion layer or, if the solubility limit is exceeded, an γ '(Fe 4 N) or ε (Fe 2-3 C x N y ) iron nitride phase with a thickness of 2 µm to 20 µm is built up. While the achievable surface hardness is essentially determined by the type of steel, the thicknesses of the layers produced are also influenced by the treatment temperature, the treatment times and the nitrogen supply in the process gas.

Process technology

In order to be able to guarantee a controlled and targeted surface refinement with the help of a plasma treatment, it is necessary to carry out the treatment process under defined conditions. The process sequences are processed fully automatically through the use of the latest process and control technology, so that no special requirements are placed on the operator. Modern systems are equipped with PC controls and field bus systems. Actual and target values ​​are visualized and documented in schematic representations. The functional positions of relevant manipulated variables are also clearly displayed. Important functions of the plasma ovens, such as pressure switches, valve positions for evacuation and treatment, provision of cooling water and the operation of the vacuum pumping station are monitored and visualized. The process gas composition is controlled by an electronically regulated mass flow controller. The pressure in the reaction chamber is recorded by pressure gauges and the pressure is set using control valves in the suction line or using speed-controlled Roots pumps. The plasma parameters pulse height, duration and cycle frequency are freely adjustable in the treatment processes. To suppress the arcs , switch-off times of less than two microseconds and pulse repetition times of up to 20 kHz are implemented.

The warm-wall or hot-wall furnace concept with additional air cooling allows a defined, accelerated heating and cooling of the batch. Standard operating modes allow treatment with constant plasma power densities. In plasma nitriding, for example, this ensures that the treatment results are independent of the furnace size and the surface to be treated. Due to the pulse technology , the energy input into the batch to be treated is significantly lower than with pure direct voltage. Therefore, very tight temperature tolerances can be maintained within the batch. Even with high charging densities, with the help of pulse technology, minimal variations in the treatment result are possible. With a constant plasma output, the temperature of the batch is usually regulated by the furnace heating, which largely decouples the thermal process control from the chemical one.

process description

The tool is connected as a cathode in an evacuated container , the container wall as an anode . A nitrogen-containing gas is supplied in small quantities. After applying a high voltage, the nitrogen atoms near the cathode are ionized. The positively charged nitrogen ions are accelerated towards the workpiece, where they hit with high kinetic energy and are embedded in the surface. The impact energy is partially converted into heat. The layer structure can be optimally influenced by precisely controlling the electrical parameters and the gas supply. If the nitrogen supply is low, it is even possible to suppress the formation of the compound layer.

Layer structure

Link layer

The structure of the outer peripheral layer portion of nitrides, the existing on carbon steels carbonitrides , nitro carbides and primary carbides in the hyper-eutectoid steel, has (engl. To the term link layer compound layer ) out. Depending on the treatment conditions and the material composition, this is a few micrometers thick. In principle, the porosity in the connecting layer cannot be avoided. It is believed that it arises because of the metastability of the Fe-N (-C) carbonitride phases. This leads to the excretion of nitrogen, which recombines to form molecules. This creates pores, preferably in energetically favored places such as z. B. grain boundaries within the compound layer.

Diffusion layer

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The edge layer area below the connection layer is referred to as a diffusion layer, sometimes also as a precipitation or mixed crystal layer. In contrast to the thickness of the connecting layer, the entire nitriding layer can reach a depth of a few tenths of a millimeter. The content of alloying elements affects the growth of the diffusion layer. With increasing alloy content, the achievable nitriding depth decreases under otherwise identical conditions.

Depending on the needs and requirements of the component, additional layers ( e.g. oxide and / or diamond-like carbon layers) can be applied.

System structure

An important feature of a so-called warm wall system is that the workpiece is mainly heated by the wall heating and not by the plasma. In this way, the plasma process parameters can be optimally adapted independently of the process temperature. The plasma voltage is applied in the form of square pulses with a repetition frequency of up to 50 kHz. Pulses of positive and negative polarity are possible, which allows the deposition of insulating layers. Thermocouples ensure accurate temperature measurement and adjustment. This is measured either close to or directly in the workpiece.

properties

Delays

Due to the low process temperatures, nitriding is the suitable diffusion method when it comes to low warpage. The lattice structure of the steel, which has been modified by the storage of nitrogen, also reduces the risk of adhesion, which leads to fretting of sliding and rolling pairs. Nitriding thus offers certain emergency running properties.

Due to the low delay, reworking is the exception. The surface growth is 10-25 µm depending on the nitriding depth. Thin-walled bodies or porous materials, however, grow more overall, but also reproducibly. This can be effectively counteracted by appropriately holding dimensional deviations.

Environmental aspect

The plasma nitriding process is one of the most environmentally friendly methods of surface hardening due to the fact that no toxic gases are used. If you compare them with gas nitriding systems, for example, the latter generate 2,700 times as much CO / CO 2 and 5,500 times as much NOx as plasma nitriding systems.

Masking

In addition to the low warpage and environmental friendliness, plasma nitriding technology also offers the option of using mechanical masking instead of masking paste . These can be easily put on and taken off again. When it comes to gaps, bores or the like, all covers can even be dispensed with, since the nitriding effect can be varied by means of the pressure. If the setting is correct, the bores will not be nitrided in order to prevent hardening or embrittlement.

Investment costs

As a rule, the investment costs of plasma nitriding systems are 20 percent to 30 percent higher than those of comparable gas nitriding systems. However, due to the lower process costs (as a result of the gas composition), this number is quickly put into perspective.

Charging

A decisive factor is that no bulk material can be treated in the plasma . The components must therefore be charged individually - whether manually or by means of a robot - with a distance between them must be maintained.

Materials and areas of application

A wide range of materials can be nitrided in plasma without any problems:

Plasma nitriding technology is used in many industries:

  • Transmission industry and its customers such as motion control , wind power or manufacturers of automobiles, trucks, tractors or construction machinery
  • Arms industry
  • Aviation (e.g. central shaft, bearings, etc.)
  • Automotive supply industry (e.g. sintered components, shafts, eccentric shafts, rocker arms, basic shafts, valves, crankshafts, pistons, gas springs, etc.)
  • Hydraulic industry (e.g. cylinders, pistons, etc.)
  • Oil and gas industry (e.g. offshore components, valves, fittings, etc.)
  • Tool making (e.g. forgings, aluminum die-cast or extrusion molds , etc.)

See also

swell

  • T. Bell, Y. Sun, A. Suhadi: Environmental and technical aspects of plasma nitrocarburising. In: VACUUM Surface Engineering, Surface Instrumentation & Vacuum Technology. Volume 59, 2000.
  • KC. Kramer, A. Mühlbauer: Practical Guide Thermoprocess Technology: Fundamentals - Processes. Vulkan, Essen 2002, ISBN 3-8027-2922-6 .
  • K. Lange: Umformtechnik: Handbook for industry and science. Volume 4: Special processes, process simulation, tool technology, production. Springer, Berlin / Heidelberg 1993, ISBN 978-3-540-55939-9 .
  • D. Liedtke: Heat treatment of ferrous materials I: Basics and applications. 7th edition, expert verlag, Renningen 2007.

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