Plasma-Assisted Nitriding Offers Production Flexibility
Plasma nitriding/ nitrocarburizing can be used to treat a wide range of materials including carbon and low-alloy steels, tool steels, stainless steels, cast irons, sintered steels, and even titanium. The IONITR plasma nitriding/nitrocarburizing process currently is used to improve the surface hardness and wear resistance of parts and components in a number of different industries including the automotive industry (gear wheels, crankshafts, dies, gear parts and synchronizing rings), plastics industry (extruders, injection molds and cylinders), general mechanical engineering (pumps and hydraulic cylinders), and metals processing industry (molds, dies and cutting and drilling tools). The IONIT OXR process combines the nitriding process with an oxidation step, which extends treatment benefits by improving corrosion resistance and tribological properties of treated parts in addition to increasing surface hardness.
Surface Hardness and Wear Protection
The plasma nitriding/nitrocarburizing process is based on a selective dispersion of nitrogen and/or carbon into ferrous materials. In the vacuum process, a low-energy plasma is used to ionize nitrogen, which then is accelerated toward the component and into the surface, diffusing into the material usually to a maximum depth of 0.8 mm (0.03 in.). The absorption of nitrogen/carbon creates a nitride surface layer in the material. The layer is a pure diffusion layer at a certain concentration of nitrogen. However, a change in nitrogen concentration (based on treatment time and temperature) results in the formation of a new ____(gamma prime) or _ (epsilon) type compound layer out of the diffusion layer (Fig. 1) to a depth between 2 and 20 _m (79 to 790 microinch).
Benefits of plasma nitrided/ nitrocarburized parts include:
- High surface hardness
- Significantly increased wear resistance
- Reduced adhesion
- Increased part service life
- Improved torsion and ductility behavior
- Improved corrosion resistance
- Improved high-temperature strength
- Little or no distortion
Corrosion resistance is achieved due to the positive influence of ferrous nitrides on the chemical reactivity potential of the material. Further advantages of the process include the good reproducibility of the nitride layer structure and only a marginal increase in part volume and surface roughness.
Plasma nitriding/nitrocarburizing can be applied on all types of steel, cast irons and sintered steels. Typical characteristics for some treated steels are shown in Table 1.
Corrosion Protection and Further Advantages
Automotive and hydraulic parts often require a combination of good corrosion resistance and low friction properties and wear resistance in addition to good surface hardness. To achieve these surface requirements, Metaplas developed a process combining plasma nitriding, gas nitriding and gas nitrocarburizing processes with a controlled oxidation process. The patented three-step IONIT OX process consists of gas nitrocarburizing, plasma activation and oxidation. The environmentally friendly process is free of any hazardous waste materials, offering an alternative to conventional methods such as chromium plating and salt-bath nitriding (Table 3).
In the gas nitrocarburizing step, both a diffusion layer and compound layer are created on the substrate using a sensor-controlled process. The surface is saturated with nitrogen, and nitride precipitation creates a high level of internal compressive stress. This produces a surface hardness greater than that produced using conventional methods, which contributes to increased part service life. The diffusion layer serves as a structural base for the subsequently formed layers. Controlled growth of a pure _-compound layer produces a defined pore structure on the surface (not inside the layer). The high surface hardness and wear protection is derived from this layer, which is about 15 to 30 _m (590 to 1180 microinch) thick and has a hardness range of about 800 to 1400 HV.
Plasma activation involves the modification of the compound layer surface. This produces a very clean surface having numerous nuclei on which to grow an extra fine, dense oxide layer.
In the oxidation step, a strongly adhering 1 to 2 _m (40 to 80 microinch) thick ferric-oxide (Fe3O4) layer is created on the surface of the modified compound layer. The strong bond is achieved due to the very good interlocking characteristics between the oxide and compound layers, wherein the oxide layer fills the pores in the compound layer.
The microstructure of a treated medium carbon steel (Fig. 2) shows the three distinct layers. The continuous compound zone has a defined microporosity toward the surface, which provides a means for the oxide layer to form a strong mechanical interlock.
Benefits of the process include:
- Good corrosion resistance
- Improved surface hardness
- Good sliding and friction properties due to a reduced friction coefficient
- Good dynamic mechanical and tribological properties
- Improved fatigue strength
- No galvanic corrosion with aluminum
- Good adhesion behavior due to ceramic-like surface characteristics
The combination of good corrosion resistance and good friction properties is especially attractive in the automotive industry. In salt-spray tests (DIN 50 021, according to ASTM B117), IONIT OX treated C35 (AISI 1035) parts remain free of the first signs of corrosion for 500 h compared with about 300 h and 280 h for hard chrome-plated (20 _m thick) and nickel-plated (20 _m thick) parts, respectively. The friction coefficient, Kr, under dry conditions is 0.35, and 0.05 under lubricated (oiled) conditions. The process also is easily adaptable to high volume production. In the hydraulic and pump manufacturing industry, piston rods, housings and pumps also benefit from the low friction coefficient, improved wear resistant properties, corrosion resistance and the smooth surface imparted by the treatment.
Plasma nitriding plus oxidation is suitable for use in a wide variety of industrial applications involving parts that require resistance to wear and corrosion caused by dynamic mechanical and friction-related stresses. Automotive parts treated using the the combination plasma-nitriding plus oxidation process include ball joints, piston rods, guide pins, selector shafts and gas-pressure springs (Fig. 3). Depending on the part, the savings in production costs vary from 30 and 60% compared with salt-bath treated and hard-chrome plated parts. Part life is extended by up to three times that of hard chrome-plated parts.
The surface treatment meets complex ball-joint performance requirements. For instance, the ball must have guaranteed resistance to both general and fretting corrosion. It also must have low frictional momentum of the system ball against plastic cup and locker ring, as well as stability against abrasive particles intruding into the ball joint. The uncovered pin and thread requires guaranteed corrosion resistance and a high endurance (fatigue) limit, and the thread must have good resistance to galvanic corrosion at the transition to the aluminum suspension arm. The treated balls meet salt-spray test requirements of 240 h minimum without any sign of red rust.
Performance requirements for guide bolts (Fig. 4) and gear-selector shafts (Fig. 5) include good general corrosion resistance, a high endurance limit for fatigue torsion loading, a low frictional momentum of the lever-plastic bushing system and resistance to galvanic corrosion between the aluminum casting and lever.
Properties of plasma nitrided plus oxidized hydraulic cylinders (Fig. 6) include improved surface behavior, such as good resistance to corrosion and resistance against intruding abrasive particles. The friction coefficient is adapted to meet the requirements of sealing and piston guidance and wear resistance against sticking dirt.
For more information: Dr. Thomas auf dem Brinke is product manager, Metaplas Ionon, AM Böttcherberg 30-38, D-51427 Bergisch Gladbach, Germany; tel: +49-02204-299-293; fax: +49-02204-299-266; e-mail: firstname.lastname@example.org