.! z _ NASA Technical Memorandum I02398 Advances and Directions of .... Ion Nitfiding/Carburizing Talivaldis Spalvins Lewis Research Center Cleveland, Ohio Prepared for the .... 2nd International Ion Nitriding/Carburizing Conference _/j --_sponsored by the American Society for Metals Cincinnati, Ohio, September 1_8-20_ 1989 .... [ ASA NQO-11902 ......... (NASA-TM-IOZ 3Q_) AnVANCU_S AND ,.qlPECTTONS OF ION NITP I9[N6/CARBU RITING (iWA:,A) 6 p CSLL 13_ Unc! _s G_13t 02392,B3 ADVANCES AND DIRECTIONS OF ION NITRIDING/CARBURIZING Talivaldis Spalvins National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio 44135 ABSTRACT 590 °C and is a ferritic treatment, whereas car- bon diffusion is performed in the austenite phase Ion nitriding and carburizing are plasma acti- between 925 to 1050 °C and is an austenitic vated thermodynamic processes for the production treatment. of case hardened surface layers not only for fer- To obtain the best response to nitriding rous materials, but also for an increasing number steels, they should contain nitride forming ele- of nonferrous metals. When the treatment varia- ments such as aluminum, chromium, titanium, bles are properly controlled, the use of nitroge- molybdenum, vanadium, and tungsten. In carburiz- nous or carbonaceous glow discharge medium ing the maximum amount of carbon that can be dis- offers great flexibility in tailoring surface/ solved in the austenite phase can be determined near-surface properties independently of the from the Fe-Fe3C phase diagram. The nitriding bulk properties. The ion nitriding process has and carburizing processes can be divided into two reached a high level of maturity and has gained broad categories: conventional and ion assisted wide industrial acceptance, while the more (plasma) processes. The conventional case har- recently introduced ion carburizing process is dening processes are performed in solid, liquid rapidly gaining industrial acceptance. or gaseous mass transfer media. These processes have been extensively described in the literature The current status of plasma mass transfer and will not be discussed in this review. mechanisms into the surface regarding the forma- The ion nitriding/carburizing processes are tion of compound and diffusion layers in ion plasma activated thermodynamic processes for the nitriding and carbon build-up ion carburizing production of case hardened surface layers not will be reviewed. In addition, the recent devel- only for ferrous metals but also for an increas- opments in design and construction of advanced ing number of nonferrous metals. The ion equipment for obtaining optimized and controlled assisted diffusion processes are based on the case/core properties will be summarized. Also, energetic nature of the glow discharge. The new developments and trends such as duplex plasma high energy of the nitrogenous or carbonaceous treatments and alternatives to dc diode nitriding plasma contains ions, electrons, radicals, and will be highlighted. activated species. The interaction between the plasma and the solid surface is based on excita- tion, ionization, dissociation and acceleration. THE CASE HARDENING PROCESSES such as nitriding, The use of the glow discharge offers great flexi- carburizing and carbonitriding are well estab- bility in tailoring surface/near-surface proper- lished thermochemical diffusion processes for ties independent of the bulk properties when the surface/near-surface hardening of ferrous treatment variables are properly controlled. materials, mainly steel. These processes are The ion nitrfding process has reached a high widely used in the manufacturing and machining level of maturity and perfection and has gained industries primarily to treat engine components, wide industrial acceptance primarily for increas- tools for hot/cold work, die castings, machine ing wear resistance and antigalling, and improv- tools, and tribocomponents. These processes are ing fatigue life and corrosion resistance. based on diffusion of the nonmetallic intersti- However, the more recently introduced ion carbu- tial elements, carbon, and/or nitrogen, into the rizing process stands on the edge of industrial surfaces. Hard surface compound nitride or car- applications. bide layers with an extended diffusion zone are The objective of this introductory paper is formed. Nitrogen diffusion in steels is usually to address the current status and understanding performed in the ferrite phase between 450 to of plasma-masstransfer processesandplasma- surface interactions as to the formationof the 2500 compoundlayer anddiffusion zoneduring case ;>---H hardening. Further, the paperwill highlight the recent accomplishmentsin equipmentdesign/ 2000 construction andthe newtrends in utilizing GLOW DISCHARGE % //_ duplex plasmaprocessesandthe developmentof CORONA I > 1500 - I 'OWNSENO.b'l I / 1 various improved/modifiedion nitriding pro- "--DISCHARGE - cesses. Somegeneralreferences(Refs. I to 5) I 0I . tl _ SUBNORMAL / / / on the subject describedhere are given in the I000 ,'" -v""---"Xd'_] GL°WD'SO'ARG_I 1 referencessection. --/ \ i NORMAl_ I / 1 , '_ _- GLOW _ / IONASSISTEDSURFACETREATMENTS i kJ ,SC.ARGEi/ / SO0 7 i ARC Theion assisted surfacemodification/ DISCIIARGE_ deposition treatmentscanbe classified in three --L_..-.-4_ I t t categories: o _o-_2 ,o-" lo-_ lo (I) Ion assisted depositionwhichcovers CURRENT, A physical vapordeposition (PVD)suchas FIGURE I. - VOLTAGE - CURRENT ClIARACTERISTICSOF DIFFERENT TYPES sputtering and ion plating, andchemical OF DISCIIARGEIN ARGON. (EDENHOFER) (REF. 6.) deposition (CVD)suchas plasmaenhanced deposition; (2) Ion beamtechniqueswhichcover ion in which the current increases with the voltage. implantation, ion beammixing, and ion In the abnormal region, the glow covers the spec- beamenhanceddeposition; and imen uniformly and therefore, uniform treatment (3) Plasma thermochemical processes such can be expected. Further, the control of this as ion nitriding, ion carburizing, ion unstable abnormal glow discharge is the critical carbonitriding, ion boriding, and ion feature in the production of uniform reproducible oxidation. layers. With the advancement of technology, new Depending on deposition energies and surface plasma power generators have been introduced to interactions, the above processes can be classi- prevent the formation of arcs, so that the glow fied as processes that produce distinct overlay discharge can be safely maintained at high cur- coatings (ion assisted deposition) and processes rent densities in the abnormal region. forming no discrete coating but which modify the The thermochemical processes such as ion surface/near surface by diffusion,'penetration nitriding are performed over a fairly well and chemical reaction (plasma thermochemical pro- defined range of pressure and voltage (typically cesses, ion implantation). In this paper, the i to i0 torr and 0.3 to 0.8 kV) under dc diode plasma thermochemical diffusion processes with plasma conditions. A typical ion nitriding sys- regard to plasma-mass transfer/interactions will tem is schematically shown in Fig. 2. be described. GLOW DISCHARGE CONSIDERATIONS GAS MIXTURE CONTROLLER ----'--"F--_ The glow discharge used as a processing NITROGEN ------_[ plasma for surface modification can be estab- ttYDROGEN --'I t----IT lished and sustained in various ways: dc diode Y°"°cAR"°"--"U II discharges, rf discharges, microwave discharges, electron emission configurations and magnetically iTr (.) enhanced discharges. In the plasma assisted diffusion treatment, processes such as ion nitriding/carburizing, the dc diode discharge is most widely used. In these nonequilibrium plasma ,,,,',",,, _ GLOW only the temperature of the electrons is high and SPECIMEN IF///J j. i "1 that of the reactive species is relatively low. (CATI,oI_E__ . !F/_I_-" / Because the temperature of the electrons is high, excitation and ionization of neutral atoms/ molecules increase, resulting in enhanced reac- tive chemical reactions and plasma heating. Since the interactions of the energetic ion spe- T F.I-IEJ cies with the surface are localized, the results which are obtained are different from that of simple (conventional) thermal activation. Several types of glow discharges depending on the relationship between voltage and current exist as shown in Fig. i. All plasma Lhermochem- ical processes are performed in the abnormal glow FIGURE 2. - ION NITRIDING SYSTEM. 2 10N NITRIDING/CARBURIZING MECHANISMS I I[ DIFFUSION LAYER GAMMA PRI/'_LAYER + EPSILON LAYER _' Presently there is no universal model which DIFFUSION LAYER DIFFUSION LAYER explains the ion nitriding process. However, numerous mechanisms have been proposed to explain this concept. The fact remains that the exact reactant species in the glow discharge during ion CORE nitriding are not well identified. As to ion carburizing, which is a more recent process, the basic explanations are derived from the more GAS COMPOSITION widely proposed ion nitriding concepts. Basically, the ion nitriding/carburizing I-5% NITROGEN 15-30% NITROGEN 60-70% NITROGEN processes are controlled by plasma mass transfer BALANCE CARRIER GAS BALANCE CARRIER GAS I-3% METIIANE (nitrogenous/carbonaceous species) and plasma- BALANCE CARRIER GAS surface interactions. All the proposed mechan- FIGURE 3. - METALLURGICAL CONFIGURATIONS DURING PLASMA NITRIDING. isms are essentially derived from these basic contributing factors: 1.2 (i) Sputtering, (2) Ion excited atom implantation, MA/CM 2 (3) Adsorption, and 1.0 (4) Condensation/deposition.