Plasma Nitrided Layer Research Articles

Structural characteristics of low-temperature plasma-nitrided layers on AISI 304 stainless steel with an α′-martensite layer

The characteristics of the structure formation of a low-temperature plasma-nitrided layer on AISI 304 austenitic stainless steel with a pre-existing α′-martensitic deformation layer were studied in detail by glancing-angle X-ray diffraction (XRD). The results show a two-step formation process for the plasma-nitrided layer. The α′-martensitic layer gradually reverts to the original γ-austenite structure at first, followed by the transformation of γ→γ N as the nitrogen content increases with treatment time. Within the same nitriding time, the sample with an α′ layer shows much less lattice expansion than the sample without an α′ layer. The volume fractions of α′ and γ phases transformed during the plasma-nitriding process were calculated by quantitative XRD. The transformation of α′→γ is dependent on the nitrogen content dissolved in α′, apart from that resulting from heating at the nitriding temperature.

Joint Second Prize Low Temperature Plasma Nitriding of 316 Stainless Steel – Nature of S Phase and Its Thermal Stability

Conventional plasma nitriding of stainless steels (at approximately 550°C) confers significant hardening of the treated case but at the expense of the corrosion resistance. Attempts have been made in the past decade to achieve combined improvements in wear and corrosion resistance of austenitic stainless steels (ASS) via low temperature thermochemical processes. However, varying degrees of success have been achieved in applications, which may be largely attributed to the lack of scientific understanding of the nature of the S phase and its thermal stability. The present work concerns two aspects of low temperature plasma nitriding of austenitic stainless steel: the study of the nature of the S phase and the thermal stability of the low temperature plasma nitrided layers on austenitic stainless steel. To achieve these tasks, systematic low temperature plasma nitriding of austenitic stainless steel has been carried out. The structure of the S phase was systematically investigated. The thermal stability of the low temperature plasma alloyed layers on stainless steel was studied employing both long term isothermal annealing and TEM in situ heating observations.

Effect of workpiece geometry on the uniformity of nitrided layers

The growth behavior of plasma-nitrided layers on workpieces with complex geometry was systematically investigated. AISI 316 stainless steel pellets with different heights were nitrided under a mixture of N 2–80% H 2 at different temperatures (673, 773 and 843 K) and pressures (100 and 500 Pa). Significant differences in thickness and hardness of the resulting nitrided layers were observed as a function of nitriding parameters. The thickness of nitrided layers increased with sample height, excepted those nitrided at 843 K. The diameter of eroded rings, commonly observed on nitrided samples, varied with coupon height. Changes in both layer thickness and eroded ring diameter are presently addressed based on the thermal balance and charge density that take place near the edges of the samples.

Hydrogen behavior in the iron surface layer modified by plasma nitriding and ion boronising

The effects of the plasma nitriding with the formation of compound nitride and diffusion zones and of the boronising with the different ion doses on hydrogen distribution and hydrogen induced deterioration of a surface layer were examined in the case of Armco iron. Electrochemical studies of hydrogen permeation rate, hydrogen vacuum extraction measurements, optical and scanning microscopy, X-ray diffraction and elastic recoil detection analysis (ERDA) were used. Accumulation of entering hydrogen within the various constitutent zones of the modified layer inhibits the hydrogen transport into the metal and thus, decreases the mean hydrogen content in the deeper zones and in the core. Hydrogen accumulation within the compact nitride zone causes the expansion of the nitride lattice, nitride phase transformation and deterioration. The ion boronising enhances the hydrogen effects in the plasma nitrided layers. Therefore, modification of the surface layer by plasma nitriding and ion boronising may result in preventing of the bulk metal from hydrogen induced degradation, but may cause hydrogen deterioration of the surface layer, depending on the treatment parameters.

Improvement of Erosion Resistance of Titanium with Different Surface Treatments

To improve the erosion resistance of titanium, several surface treatment methods were applied: (1) duplex treatment with carbon nitride film deposited on a plasma-nitrided layer, (2) diamond coating, and (3) laser alloying. Duplex treatment could improve the erosion resistance of titanium under a low impact velocity of erosion particles. However, under a high velocity of erosion particles, because of the shallow depth of the plasma-nitrided layer and low load-bearing capacity of carbon nitride layer on the plasma-nitrided specimen, the improvement of erosion resistance was not significant. Diamond coatings with a thickness of 15 µm made no significant improvement on the erosion resistance of the titanium substrate. The large-area spallation of diamond coating during erosion was observed, probably due to the high residual stresses, poor load-bearing capacity, and brittle nature of diamond coatings. Compared with untreated Ti substrate, the erosion resistance of the laser-alloyed (nitrided) specimen was improved significantly. The erosion mechanisms for laser-nitrided titanium were characterized by chipping, brittle fracture, and formation of large flakes in the laser-nitrided layers.

Indentation and Scratch Tests on Sputtered Amorphous CN<SUB><I>X</I></SUB> Films Deposited on Plasma-Nitrided Ti-6Al-4V

Sputtered carbon nitride (CNX) films were deposited on both untreated and plasma-nitrided Ti-6Al-4V substrates. Surface and cross-section morphology of the deposited CNX films was studied by scanning electron microscopy (SEM). Modified Vickers hardness tests showed that the intrinsic hardness of the CNX film was about HV 2000 to 3000. Both the indentation and scratch tests showed that, compared with the CNX film deposited on Ti-6Al-4V substrate, the load-bearing capacity of CNX film deposited on a plasma-nitrided layer was improved dramatically. From the results of scratch tests, the duplex-treated system was effective in maintaining a favorable low and stable coefficient of friction and improving the wear resistance of Ti-6Al-4V substrate.

Hydrogen embrittlement of titanium during microwave plasma assisted CVD diamond deposition

During diamond deposition on a titanium substrate using a gas mixture of H2–CH4 (196 : 4), hydrogen easily diffused into the substrate and led to significant microstructural coarsening and a severe loss in Charpy impact energy. In order to prevent the rapid diffusion of hydrogen into the substrate during diamond deposition, three techniques were studied. The first method was to use a post-vacuum annealing treatment to achieve dehydrogenation of a diamond coated titanium specimen. However, the Charpy impact energy could not be restored significantly even after a few hours of annealing. The second method was to apply a barrier interlayer between the diamond coating and titanium substrate. Results showed that even though the sputtered TiN coating and plasma nitrided layer could prevent the rapid diffusion of hydrogen and carbon into the titanium substrate, the deposited diamond coatings had poor adhesion to the substrate. A graded interlayer produced by plasma nitriding followed by plasma carbonitriding was effective in preventing the rapid diffusion of hydrogen and also improving the nucleation rate and adhesion of the diamond coating. The third method was to use a gas mixture of Ar–H2–CH4 instead of the conventional H2–CH4 . With the use of an Ar–H2–CH4 (180 : 16 : 4) mixture, a smooth and nanocrystalline diamond coating was deposited, and there was little change in the substrate microstructure or Charpy impact energy after diamond deposition.

Wear behaviour of plasma nitrided steels at ambient and elevated temperatures

The wear behaviour of plasma nitrided and white layer removed from AISI H13 and 722M24 steels were investigated at elevated temperatures. The samples prepared from the test materials plasma nitrided at 530–590°C for 4–100 h were tested at temperatures of 50–200°C under lubricated abrasive conditions. The wear behaviours of the samples are completely different in the test conditions from those observed at room temperature and dry conditions. It was observed that wear rate and surface roughness were decreased by the test temperature due to the softening of nitrided layer although the friction coefficients were increased under the test conditions. The removal of white layer produced during the plasma nitriding help the increase of wear resistance. On the other hand, abrasive particles in lubricant penetrate into the softer surface and protect the surfaces against abrasion. The solidified oil layer by the test temperatures also increased the wear resistance of the steels providing easy sliding.

Effects of pre-treatments and interlayers on the nucleation and growth of diamond coatings on titanium substrates

During diamond deposition on titanium substrates, two processes exist: (1) diffusion of hydrogen into a titanium substrate and the formation of hydride thereby degrading the mechanical properties of the substrate; and (2) competition among the rapid diffusion of carbon atoms into substrates, the formation of carbide and the nucleation of diamond crystals (thereby affecting the nucleation and growth rate of the diamond coating). To increase the diamond nucleation rate and prevent the rapid diffusion of hydrogen and carbon into the substrate, different surface treatments and interlayers were studied in this paper. Results showed that polishing with diamond pastes and ultrasonic pre-treatment in diamond suspensions will significantly increase the nuclei density of diamond crystals. However, the diffusion of hydrogen into the substrate could not be prevented . Pre-etching of the titanium substrate using hydrogen plasma for a short time significantly increased the nuclei density of diamond crystals. Results showed that on a TiN interlayer, there was no significant improvement in diamond nucleation and growth, and the deposited diamond coatings showed poor adhesion. New diamond crystals were formed on the DLC interlayer in which DLC acted as the precursor for diamond nucleation. However, the so-formed diamond coating showed spallation. The plasma nitrided layer could prevent the rapid diffusion of hydrogen and carbon into the titanium substrate, but results showed a relatively low nucleation density of diamond crystals and poor adhesion. A graded interlayer combining plasma nitriding followed by plasma carbonitriding was effective in preventing the rapid diffusion of hydrogen and carbon into the substrate and improving the nucleation rate and adhesion of diamond coating.

Study of microstructure of low-temperature plasma-nitrided AISI 304 stainless steel

The microstructure of the low-temperature plasma-nitrided layer on AISI 304 austenitic stainless steel was studied by transmission electron microscopy (TEM). The results show that the surface of the layer consists of a supersaturated solid solution ({gamma}{prime}{sub N}) based on the {gamma}{prime}-Fe{sub 4}N phase whose electron diffraction pattern (EDP) has a strong diffuse scattering effect resulting from supersaturating nitrogen (above 20 at. pct) and {l_angle}110{r_angle} streaks arising from matrix elastic strain due to the formation of paired or clustered Cr-N. The latter is due to the N above to 20 at. pct {gamma}{prime}-Fe{sub 4}N-phase value and leads to a lattice parameter that is greater than that of the {gamma}{prime}-Fe{sub 4}N phase. The subsurface of the layer is composed of a supersaturated solid solution based on {gamma}-austenite, which is an expanded austenite, {gamma}{sub N}. Its morphology shows the basketweave or tweedlike contrast consisting of so-called stacking fault precipitates having twin relationships with the matrix whose EDP shows diffuse scattering streaks with certain directions. The {epsilon} martensite transformation was observed in the subsurface of the layer. The increase in stacking faults compared with the original stainless steel and formation of {epsilon} martensite in the subsurface of the layer indicate that nitrogen lowers themore » stacking fault energy of austenite.« less

Low pressure plasma arc source ion nitriding of austenitic stainless steel

A series of low pressure plasma arc source ion nitriding experiments has been conducted on AISI 304 stainless steel at temperatures ranging from 350 to 460°C in an N 2–H 2 gas mixture. The structure and composition of the plasma nitrided layers were studied by optical metallography, X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The thickness of the nitrided layer depends on the plasma nitriding temperature. After 2 h of nitriding at 420°C, a 12 μm nitrogen supersaturated austenite nitrided layer with nitrogen content up to 45 at% on the surface was formed. The hardness values of the nitrided layer measured are in the range of 4.6 GPa for the sample nitrided at 350°C to 12.0 GPa for the sample nitrided at about 420°C.

Friction and wear behaviour of carbon nitride films deposited on plasma nitrided Ti–6Al–4V

Amorphous carbon nitride (CN x ) films were deposited on plasma nitrided Ti–6Al–4V substrate in order to improve the adhesion strength and tribological behaviour. Scratch and ball-on-disk wear tests were performed to evaluate the load bearing capacity, wear and friction characteristics of the duplex-treated coatings. Compared with a CN x film deposited on Ti–6Al–4V substrate, the load bearing capacity of a CN x film deposited on plasma nitrided layer was improved dramatically. Results showed that under dry sliding condition, the duplex-treated system was more effective in maintaining a favourable low and stable coefficient of friction and improving the wear resistance than both individual plasma nitriding and CN x film on Ti–6Al–4V substrate. The reasons for this significant improvement in tribological behaviour with the application of duplex treatment can be attributed to the combined benefits from both plasma nitriding and CN x films. (1) Plasma nitriding of Ti–6Al–4V produces a graded hardened case which serves as an excellent supporting and load bearing layer for hard CN x films. (2) The CN x film deposited at low temperature can produce a wear resistant and low-friction surface without impairing the beneficial effects from plasma nitriding treatment, and the smooth CN x films could effectively reduce both the interfacial stresses and the stresses near the surface thus providing a good tribological behaviour. (3) The graphitisation of the wear debris during dry sliding condition can help to decrease the coefficient of friction and improve the wear resistance.

Characterization of plasma nitrided pure titanium by X-ray absorption spectroscopy

To date, the exact nature of the plasma nitriding mechanism and the role of energetic particle bombardment are not well understood. The purpose of this work has been to obtain a more detailed knowledge about the evolution of the plasma nitrided surface layer as a function of the energy of the bombarding particles. Nitrided layers were produced at the surface of pure titanium specimens at various flux energies by Intensified Plasma-Assisted Processing (IPAP), a triode plasma technique developed in our laboratory. X-ray Absorption Near Edge Structure (XANES) spectroscopy and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy were used to characterize the local structure of the titanium nitride layers. Cross sections of the processed specimens were studied by Auger electron spectroscopy and electron microscopy. The results showed that increasing flux energy promotes the formation of a well-ordered TiN layer at the surface. Low flux energies produce significantly lower fractions of the TiN phase at the surface, as well as thinner nitrided layers. A structural model was suggested and quantitatively tested based on the XANES and EXAFS measurements.

Plasma-nitriding of tool steels for combined percussive impact and rolling fatigue wear applications

Plasma-nitriding is a flexible, multifunctional case-hardening technology. In the present investigation, this technique was applied to increase the compound percussive impact and rolling contact fatigue resistance of steel-plant hot-rolling-mill components, made out of AISI H13 and D2 grades of tool steel. Plasma-nitrided layers were characterized by optical and scanning electron microscopy, microhardness measurements, and phase analysis and residual stress determination using X-ray diffraction techniques. The service life of the components was determined in the real plant operation. Plasma-nitriding improved the service life by two- to 3.6-fold, depending upon the type of steel and the components' contact surface geometry. Redesigning of the surface/subsurfaces by plasma-nitriding coupled with appropriate selection of substrate enhanced the service life of the component by up to sevenfold. The service life was examined in relation to nitrided layer characteristics, and the importance of residual stresses and surface microstructure are highlighted.

Sliding wear characteristics of low temperature plasma nitrided 316 austenitic stainless steel

The sliding wear behaviour of plasma nitrided layers produced in AISI 316 type austenitic stainless steel at temperatures between 450°C and 550°C has been investigated in the present work. Both dry sliding test with bearing steel and alumina as the counterface and sliding test in a corrosive solution have been conducted. The results show that the sliding wear behaviour of plasma nitrided 316 steel depends on the nitriding temperature, counterface material and testing conditions. Under dry sliding conditions, plasma nitriding at various temperatures increases the wear resistance of 316 steel by more than two orders of magnitude when sliding against bearing steel. When sliding against alumina under dry conditions, the low temperature (450°C) nitrided layer, which comprises predominantly a single ‘S’ phase, is more effective in improving the wear resistance of 316 steel. When sliding in a corrosive solution, no improvement in corrosion wear resistance has been observed for the high temperature nitrided layers, whilst the low temperature nitrided layer, as compared with untreated 316 steel, exhibits not only improved corrosion resistance but also much increased corrosion wear resistance under the present testing conditions. Accordingly, combined corrosion and wear resistance of 316 steel can be achieved by plasma nitriding at low temperatures to produce a thin and precipitation-free layer with high hardness and good corrosion resistance.

Effect of silicon manganese and nickel on plasma nitrided layer on sintered steels : A.N. Klein et al (LABMAT/UFSC, Florianopolis, Brazil)

Effect of silicon manganese and nickel on plasma nitrided layer on sintered steels : A.N. Klein et al (LABMAT/UFSC, Florianopolis, Brazil)

TVibological Behavior of TiN and TiAIN Deposited on Substrates Plasma Nitrided at Low Pressure

Abstract Binary and ternary compounds of TiN and (Ti,Al)N were deposited by magnetron sputtering over low pressure plasma nitrided layer. Tribological behavior under dry-sliding conditions was evaluated with pin-on-ring test machine. The significant process parameters, friction coefficient and contact temperature, were checked with a modern measurement line that includes computer for acquisition and processing of data and monitoring the wear process. The wear zone morphology and characteristics of surface layer structure as well as important properties were investigated by scanning electron microscopy (SEM). Energy-dispersive x-ray analysis (EDAX) of the wear-scars on pins provided essential information on the wear characteristics. Based on all results the correlation between the surface structure and tribological wear characteristics were explained. It was concluded that formation of the plasma nitrided layer at low pressure, beneath a TiN and (Ti,Al)N over coating, is important in determining the use of hard coating for reducing the wear. An excellent coating to substrate adhesion and low friction coefficient was found to be significant factor influencing the use of plasma nitriding at low pressure.

Properties of TiN coatings deposited onto hot work steel substrates plasma nitrided at low pressure

The properties of the surface structure of a plasma nitrided layer beneath a TiN overcoating were investigated. The substrates were AISI H11 grade steel quenched and tempered according to the manufacturer's recommendation. Prior to deposition of the coating the substrates were plasma nitrided at a pressure of 5 Pa in a specially designed sputter ion plating unit with an auxiliary anode. The same unit was used for deposition of the TiN coating and plasma nitriding of the substrates. Several substrate induced effects were found to be caused by the previous plasma nitriding of substrates. An excellent coating to substrate adhesion was found by measuring the critical load as defined in the scratch test method. Two modes of coating failure, cohesive and adhesive, were detected and the corresponding critical loads measured. Adhesive failure of the coatings starts at loads in excess of 100 N. The substrate induced preferred orientation of the TiN coating was analyzed by X-ray diffraction of non-nitrided, conventionally plasma nitrided and low pressure plasma nitrided substrates. It is shown that the preferred orientation of coating growth changes from (111) to (220). The conventionally plasma nitrided/TiN coating composite may be considered as intermediate between non-nitrided and low pressure plasma nitrided layers with deposited TiN coating. Compressive stresses were found in both the plasma nitrided layer and the TiN coating.

Friction and wear behaviour of plasma-nitrided layers on 3% Cr-Mo steel

The tribological behaviours of plasma-nitrided layers produced on 722M24 steel in an ammonia environment have been investigated. The microhardness of the layer was measured, and the white layer and the wear debris were analysed by X-ray diffraction. The wear tests were carried out under dry and abrasive-containing lubricated conditions. The weight loss has been determined as a function of load and test time. The surface micrographs have been examined by scanning electron microscopy. Untreated samples were used as a reference. In the present tests a wear machine which allowed a disc-disc contact was used. The test results indicated that the wear resistance of 722M24 steels can be significantly improved by means of plasma nitriding at high temperature for a sufficiently long time. There are linear relationships between the weight loss and test time and effective load.

Properties of sputtered stainless steel-nitrogen coatings and structural analogy with low temperature plasma nitrided layers of austenitic steels

Structural analogies are revealed between 310 stainless steel-nitrogen deposits prepared by reactive sputtering and diffusion layers resulting from low temperature plasma nitriding of the same grade of stainless steel. Both the coatings and the diffusion layers show high nitrogen supersaturations in the f.c.c. lattice of austenite and present a rather favourable compressive stress and a good level of microhardness. An interesting corrosion resistance is also expected. With these properties, future applications can be considered.