There are some similarities between the reaction of nitrogen and oxygen with titanium, and the surface layers may well be a mixed oxy-nitride, denoted as TiX, where X represents a variable mixture of oxygen and nitrogen. This is an environmentally friendly process, using clean gases (mainly pure hydrogen, nitrogen, and argon), which requires only low amounts of energy. At the end of the process cycle, the power was switched off and the specimens were slowly cooled to room temperature in the nitriding chamber. Parts to be nitrided are cleaned, degreased and are then charged in the vessel, which is then evacuated to 0.05-0.10 torr. This ultimately leads to hollow cathode discharge. The plasma is created between the part as cathode and the chamber wall as anode. Figure 11.3. The main advantages of plasma nitriding over conventional nitriding processes are: reduced cycle time, controlled growth of the surface layer, elimination of white layer, reduced distortion, no need of finishing, pore-free surfaces and mechanical masks instead of copper plating. The plasma process operates at temperatures between 400 and 800 °C and the treatment is generally implemented by batch. Ionitriding (Plasma), or Glow-Discharge Nitriding: It is a case-hardening process of enriching the surface of the steel with nascent nitrogen by glow discharge method. Compared to hardening, plasma nitriding of the substrate improves coating to substrate adhesion and especially the load carrying capacity of the substrate. The process was not introduced into the United States until after 1950 and has only been used as a production process for the past 20-25 years. The parts are placed in a vacuum chamber and the furnace is filled with process gas containing N2 and H2 to a pressure of 100–800 Pa. The decomposition of the ammonia to release both nitrogen and hydrogen diffusion is very similar with each of the above methods except with the Plasma nitriding. Case-hardened gears provide maximum surface hardness and wear resistance and at the same time provide interior toughness to resist shock. The plasma nitriding process began in the 1920's as an alternative to conventional gas nitriding. Gas nitriding is only applied, as a rule, for alloyed steels. In this technique the glow discharge phenomenon is used to introduce nascent nitrogen to the surface of a gear and its subsequent diffusion into subsurface layers [37]. Plasma nitriding can be further branched out into plasma nitrocarburising. The excess nitrides diffuse into the gear material during the heat-treating cycle, leaving a white layer on the top surface. The well-known fatigue issue during prolonged heat treatments on titanium will be minimized, too. Its cost is considerably less than induction hardening. The formation of nitrides and microhardness, however, were optimum for nitriding at 550 °C. Due to the ion bombardment via the plasma and potential difference, both the nitrogen diffusion mechanisms and the parabolic law of layer growth are changed (56–59). Under these conditions a very hard nitrogen-rich layer some 2–10 μ m thick, consisting of assorted FexN (x = 2–4) compounds, forms within a diffusion zone a few hundred microns deep. 8.03.4.1 Plasma Nitriding. F. Ashrafizadeh, in Surface Engineering of Light Alloys, 2010. Instead, the atoms of nitrogen and carbon that deposit on metal surfaces modify them by diffusing into the underlying matrix. Accurate analysis of the diffraction patterns of the nitrided surface in Fig. 6.12B where only the flank is hardened, leaving the root area untreated [45]. Ion (Plasma) Nitriding is a diffusion process of nitrogen into the steel, that is used to improve the surface hardness, wear resistance, fatigue life, corrosion resistance of parts made of steel, cast iron, titanium and aluminum alloys, sintered materials, etc. Figure 6.11. The main advantages of plasma nitriding over conventional nitriding processes are: reduced cycle time, controlled growth of the surface layer, elimination of white layer, reduced distortion, no need of finishing, pore-free surfaces and mechanical masks instead of copper plating. This method is generally not applicable for a tooth size finer than 16 DP. Plasma Nitriding Furnaces. D. Höche, ... P. Schaaf, in Laser Surface Engineering, 2015. Other gasses like hydrogen or Argon are also used. Plasma nitriding process techniques can also be known also as Glow discharge nitriding, or Plasma nitriding Continuous DC nitriding, Pulsed Plasma nitriding. CDS is a cathode dark space and NG is a negative glow. Indeed, Argon and H2 can be used before the nitriding process during the heating up of the parts in order to clean the surfaces to be nitrided. The Ion Nitriding Process. Convective effects can influence the process, determine the surface quality, and affect the material transport. Higher surface, case, and core hardness than gas nitriding. Plasma nitriding normally occurs at temperatures of 450 to 600 °C in a vacuum with the aid of plasma generated by a glow discharge on the workpiece surface. They not only improve hardness, wear resistance, and fatigue strength, but also control friction, reduce adhesion, and improve lubrication and corrosion resistance. While an excessively hardened depth may make the gear-tooth too brittle to withstand tensile stresses, the exact required value of the case depth depends on the gear-tooth form and the application requirements. The process of plasma carburizing is carried out in an Ar/H2 atmosphere containing CH4 or C3H8 reactants at pressures between∼3 and 20 torr at temperatures of∼1000°C. Nitriding is a low temperature heat treatment process that diffuses Nitrogen into the surface of a metal to create a hard surface layer. The properties obtained in this duplex treatment were much better than those obtained by either nitriding or ion plating. Plasma nitriding is the most recent and modern form of nitriding. The depth to which the heated zone extends controls the surface hardness and case depth, and depends on the frequency of the current and on the duration of the heating cycle. Plasma Nitriding in general is a heat treating that is used to case harden surfaces using nitrogen. The glow discharge is influenced by pressure (Ruset, 1991) and the radiant heat losses, which are controlled by proper shielding (Ruset, 1994). The ionization in the gas is increased by faster electrons from one CDS that enter the other (Engel, 1965). Low Temperature Plasma Nitriding is a recent development in the nitriding of stainless steels and iron superalloys. Nevertheless, the process is very sensitive to the huge amount of process parameters and is hard to control. This process is mostly used on materials such as low-carbon, low-alloy steels and other medium and high carbon steels. free of toxic salts, ammonia and any other toxic gases) Phase controllable compound zone (or white layer) Less distortions than gas nitriding because plasma nitriding is performed at a lower temperature and under a vacuum. The process of the plasma-nitriding of gears is carried out in a furnace where an electrical voltage is applied between the gear to be treated, as the cathode, and the furnace as the anode (see Fig. In the case of duplex treatment such an intermediate compound layer may reduce the hardness and stress gradient between the substrate and hard coating, leading to superior sliding wear properties of the composite. High-density radio-frequency and direct current (RF/DC) plasma nitriding process was applied to synthesize the nitrided AISI304 microstructure and to describe the essential … There are various temperature controlling devices used in plasma-assisted techniques, including infrared thermometers, thermocouple wires under high voltage of the glow discharge, and thermocouple wires protected from high voltage. E. Roliński, in Thermochemical Surface Engineering of Steels, 2015. Plasma nitriding is a method of surface modification using a glow discharge technology to introduce nitrogen into the surface of a metal, which subsequently diffuses into the material (6). Considering the extensive demands in surface engineering, pulsed laser nitriding becomes an alternative way, offering some advantages but also disadvantages. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780857095923500033, URL: https://www.sciencedirect.com/science/article/pii/B9780857090157500075, URL: https://www.sciencedirect.com/science/article/pii/B9780080965321009122, URL: https://www.sciencedirect.com/science/article/pii/B9780857095923500112, URL: https://www.sciencedirect.com/science/article/pii/B9781855737426500087, URL: https://www.sciencedirect.com/science/article/pii/B9780125249751500094, URL: https://www.sciencedirect.com/science/article/pii/B9780128044605000067, URL: https://www.sciencedirect.com/science/article/pii/B9781845695378500105, URL: https://www.sciencedirect.com/science/article/pii/S016789220080156X, URL: https://www.sciencedirect.com/science/article/pii/B9781782420743000027, Thermochemical Surface Engineering of Steels, 2015, Process technologies for thermochemical surface engineering, Thermochemical Surface Engineering of Steels, Laser surface modification of steel for slurry erosion resistance in power plants, Laser Surface Modification of Alloys for Corrosion and Erosion Resistance, Plasma-assisted nitriding and nitrocarburizing of steel and other ferrous alloys. The mechanism of heat transfer during heating of the cathode in a glow discharge is very complex. In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. It is frequently used for forging dies or casting molds to raise resistance to wear and thermal fatigue. The surface of the heated piece is additionally heated by the plasma. It allows – vacuuming and change of the working atmosphere, ion bombardment that creates a sputtering effect and final cleansing of the parts, heating and nitriding (nitrocarburizing). Schematic of plasma nitriding of a gear. A special technical feature of this process is the possibility of using mechanical masking to provide accurate partial nitriding. In order to maintain its original dimensions the core keeps the nitrided case in compression. For carburizing and nitriding, the difference is clear. In common, these processes all rely on the undersized atoms of N, C, and B. Alloy steels with more than 0.5% carbon are susceptible to cracking [1,41]. The improved tribological properties are mainly due to the very good frictional characteristics and high hardness of the coating. The variation in the process efficiency is not only related to the pulse duration. Shivamurthy, ... G. Padmanabham, in Laser Surface Modification of Alloys for Corrosion and Erosion Resistance, 2012. Plasma carburizing using methane is a special process for partial hardening and carburizing of internal bores. First, the vessel is evacuated. 105 York Street Upon reaching the desired vacuum, the unit is back-filled with a process gas to begin the preheating cycle. The plasma cannot develop under the cover and therefore the covered surface remains free of carburizing. To achieve greater depth of heat penetration, low-frequency current is used, whereas heat treatment at shallow depth requires high-frequency current [1,41]. Plasma nitriding is a thermochemical process for enriching the surface layer of a workpiece or component with nitrogen. The plasma nitriding process began in the 1920's as an alternative to conventional gas nitriding. The applied voltage (300–800 V) accelerates the ions towards the surface of the part. According the binary phase Ti. The potential drop across this space is called the cathode fall (Brown, 1994). By adding CH4 and CO2 to the gas mix, carbo-nitriding of surfaces can be achieved. These methods make use of applied energy from external sources to harden the gears. The remaining energy is transferred either by neutrals bombardment or by radiation from neutral gas collisions in the negative glow. (B) Flank method. Overlapping of the discharge zones may create non-uniform heating. A thin oxide film is always present on the surface of titanium metal. Besides nitriding, the use of carbon for alloying according a laser carburization has been investigated as well and will be described now. As cold-wall furnaces require a minimum energy supplied by the plasma for heating and maintaining temperature, there are restrictions on the voltage and current that can be set in order to obtain the desired nitrided layers. Case-hardening produces a hard, wear-resistant case, or surface layer (to increase pitting resistance and bending strength) on top of a ductile and shock-resistant interior also known as core, of hardness 30–40 HRC to avoid tooth breakage [1]. In the plasma nitriding and carburizing processes treated in this section, nitride and carbide films are not deposited. The usual range of carbon content required in gear steels for flame hardening is 0.40–0.50%. Surface hardness and wear rate of the specimens after duplex treatment. Due to its improved capabilities, this method has become increasingly popular. It is, then, the formation of hard compounds accompanied by a graded interface in this duplex treatment that must be responsible for the improved tribological properties. Some of these methods are discussed in the following section. Plasma nitriding equipment consists of a vacuum vessel (retort), a pumping system to control the furnace pressure, a plasma generator and a cooling system. Spin hardening is best suited for gears with enough mass to absorb the excessive heat applied in this method without too much distortion. Typical micrographs of duplex treated (PVD titanium and plasma nitrided) aluminium alloy; (a) fracture section, (b) coating surface, (c) coating-substrate interface in a polished and etched section. This ionized gas collides with the product removing impurities from the surface and preparing the product for the nitriding process to begin. Among the advantages claimed for such plasma treatment are enhanced mass transfer of carbon and prevention of surface and intergranular oxidation, a feature that confers greater resistance to mechanical fatigue damage. The plasma is ignited between the furnace wall and the screen (metal mesh), which acts as a cathode. The mechanism of the case-hardening process includes a decomposition of solid, liquid, or gaseous species, the splitting of gaseous molecules to form nascent atoms, the absorption of atoms, their diffusion into a metallic lattice and reactions within the substrate structure to modify existing or form new phases. CDS becomes larger when the pressure of the discharge decreases. CDS is the most important region of the glow discharge because glow discharge cannot be maintained without it. It was determined that ion bombardment of the cathode is responsible for the transfer of about 15% of heating energy (Marciniak and Karpiński, 1980; Marciniak, 1983). R.C. It has been claimed that these methods not only increase the wear resistance and fatigue strength of gears, but also reduce gearbox noise [40]. Pulse durations within the range of microseconds are again different in their performance. The gear is placed inside a coil, and when a high-frequency alternating current is passed through the coil, rapid heating takes place due to electromagnetic induction. Under these conditions the solution to Eq. With the nitriding furnaces fully automated, we are able to run 24/7. While increasing bias voltage clearly leads to faster compound layer growth, the total case is not affected. After World War II the plasma nitriding process received widespread acceptance in Germany, Russia, China and Japan. 10.19. When the preset heating time has elapsed, the product is subjected to an ion bombardment to clean impurities from the surface. 1-24 is. Case-hardening is regarded as thermomechanical treatment to modify the surface properties of gear-teeth. Plasma nitriding is an alternative to ALLNIT ® low-pressure nitriding that BMI offers with its range of B5_R vacuum furnaces. The nitrided layer has a diffusion zone and a compound zone, i.e., a white layer. A glow discharge with a high ionisation level (plasma) is generated around the parts. This, consequently, improves the tribological performance, load-carrying ability, and smooth motion transmission characteristics of gears, with an appreciable reduction in noise, vibration, and wear rate. Gears made of cast irons (ductile, malleable, and gray), and low-alloy and medium-carbon steels with 0.4–0.55% carbon content (i.e., AISI 4140, 4340, 1045, 4150, 1552, 5150) are commonly heat treated by induction hardening process. Flame hardening is a heat treatment process where oxyfuel gas flames are directly impinged onto the gear-tooth surface area to be hardened which is then subjected to quenching. However, Mann (2000) reported negligible erosion improvement of plasma nitrided 13Cr–4Ni steel compared with as-received 13Cr–4Ni steel, but better erosion resistance compared with D-gun (Cr3C2–25NiCr). is linearly dependent on j. For partial carburizing, the plasma effect may be prevented by covering with metallic conducting masks or sheet metal where it is not required. Table 2.2. As depassivation of the surface is a key factor in controlling the extent of nitriding, it follows that if the process chamber is free from oxygen, nitriding can proceed at lower temperatures, producing both oxide and nitride within the top layers (Miyagi et al., 1980). Coatings of titanium, deposited onto the aluminium substrate, were subjected to a series of plasma nitriding cycles in order to form a graded interface and hard metal compounds on the surface. Due to the ultra-short interaction time direct ablation takes place arising in insufficient nitrogen uptake. (2005) have investigated the DC pulsed plasma nitrided 4140 steels and found that the process improved wear resistance. During plasma nitriding, a hard case at the surface of the material is formed. The process is accomplished by hardening one tooth root at a time. Laser pulses usually will be classified by their timescales and energy densities. The microstructure developed according the solidification conditions in needle-like TiN phases and dendrites. This process is time-consuming; however, it is used to harden large gears because their heat treatment by conventional processes requires a large amount of power and very large diameter gears are not fit inside any existing carburizing furnaces. Induction hardening of gears is done by two methods: spin hardening and tooth-to-tooth or contour hardening [1]. There are two ways of heating gear-teeth. The sand-blasted sample with a higher emissivity reached a lower temperature than the ground sample with a lower emissivity. Grün and Günther, 1991; De Sousa and Alves, 1997, Marciniak and Karpiński, 1980; Marciniak, 1983, The Science and Technology of Materials in Automotive Engines, Materials Science of Thin Films (Second Edition), Advanced Gear Manufacturing and Finishing. Benefits of plasma nitriding over traditional carburizing and nitriding include: Improved control of case thickness and greater uniformity; No distortion, therefore no postprocessing is required; Can be performed at relatively low temperatures (450–550°C); Improved process time compared with tradition nitriding. Plasma nitriding, known also as ion nitriding is a form of case hardening process. The processing time is dependent on the composition of the steel being nitrided and the required case depth. The rise in current density begins when two negative glows coalesce and light intensity from the glow increases at the same time. There are two types of glow discharges, which are commonly called normal and abnormal glow. 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