Conventional DC Plasma Nitriding Research Articles

Novel high-efficiency plasma nitriding process utilizing a high power impulse magnetron sputtering discharge

Lifetime and biocompatibility of orthopedic implants are crucial in meeting the new challenges brought about by the fall in the patient age and the aging population. The high-load surfaces in contact with the biological environment must display enhanced tribological properties, biocompatibility, and reduced metal ion release in long-term clinical performance. Surface modification techniques such as nitriding can significantly improve the in-service behavior of the medical-grade alloys in current use. We report on a novel approach for nitriding of CoCrMo alloys using high power impulse magnetron sputtering (HIPIMS) discharge. The new nitriding process has been successfully carried out at the National HIPIMS Technology Centre at Sheffield Hallam University, UK, in an industrial size Hauzer 1000-4 system enabled with HIPIMS technology. While the nitriding ion flux is controlled by the HIPIMS magnetron plasma source, the ion energy can be independently set via the substrate bias. Implementing the HIPIMS source allows reducing the operational pressure by one order of magnitude compared to conventional dc plasma nitriding (DCPN). Plasma analyses have identified significantly enhanced production of ions of molecular nitrogen (N2+), atomic nitrogen (N+), and N2H+ radicals in the HIPIMS discharge compared to DCPN. Because of the low pressure of operation of the HIPIMS process, the energy of ions is similar to the bias voltage, whereas the high pressures used in DCPN cause severe losses in ion energy due to scattering collisions within the sheath. The high flux and high ion energy are primarily responsible for achieving a fourfold increase in process productivity as compared to state-of-the-art plasma nitriding processes. The nitrided surface layers exhibit excellent mechanical and tribological properties, which bring about significant improvements in hardness, fracture toughness, and wear. The protective function of the nitrided layer against corrosion in the aggressive environments of simulated body fluid is remarkably augmented. The barrier properties of the nitrided layer have been demonstrated through a reduction in metal ion release by as much as a factor of 2, 4, and 10 for Co, Cr, and Mo, respectively.

Use of a solid carbon precursor for DC plasma nitrocarburizing of AISI 4140 steel

A new surface treatment concept of solid carbon source plasma nitrocarburizing (SCS-PNC) was proposed. Carbon fiber carbon composite (CFC) was used as a solid carbon precursor for conventional DC plasma nitriding process in a nitrogen-hydrogen plasma. The effect of plasma power on the phase composition of the compound layer on AISI 4140 steel was investigated. The results of the SCS-PNC process were compared to conventional DC plasma nitriding (PN) and plasma nitrocarburizing (PNC) using methane as carbon-bearing gas. The SCS-PNC process provides increasing carbon concentration within the compound layer with increasing plasma power density. In the compound layer a constant carbon content of 1.5 wt.-% C was obtained for high plasma power densities and the ε-carbonitride Fe2-3(C,N)x formation was significantly promoted reducing the porous seam zone. Moreover, precipitation of unwanted cementite Fe3C in the compound layer was effectively suppressed for SCS-PNC compared to treatment by PNC with methane addition.

Cathodic Cage Plasma Nitriding of Ti6Al4V Alloy

Glow discharge nitriding is being used increasingly more often for modifying the properties of titanium and its alloys with the aim to increase their frictional wear resistance, fatigue strength, and, in the case of medical applications, to eliminate the metallosis effect. Unlike PVD methods, ion nitriding ensures the formation of diffusive layers with very good adhesion to the substrate, but which still have some disadvanteges such as the “edge effect” or “hollow cathode effect” which hinders treatment of complex workpieces. The paper compares nitrided layers produced on Ti6Al4V alloy using two different types of nitriding processes. The first process is conventional dc plasma nitriding (DCPN) where the samples were placed at the cathode potential, while the second one is a new method of cathodic cage plasma nitriding (CCPN) process, where the substrate is insulated from the cathode and anode. The experiments have shown that the treatment conducted in a cathodic cage can be alternative for conventional ion nitriding, especially when used for small parts with complicated shapes used in the space or medical industry. DOI: http://dx.doi.org/10.5755/j01.ms.22.1.7343

Surface modification of Ti-6Al-4V alloy by cathode assiting discharge setup and conventional plasma nitriding methods

In order to improve the wear resistance of titanium alloys, the potential of the plasma nitriding process with a cathode assiting discharge setup was examined. Ti6Al4V alloy was nitrided by the cathode assiting discharge nitriding (CAN) and conventional DC plasma nitriding (CPN) methods, respectively. The micro-topography, phase composition and cross-sectional microhardness distribution of two nitriding layers were studied comparatively. The sliding wear resistance of the nitriding layers was examined by a ball-on-disk test against Si3N4 balls. The results indicated that the surface roughness Ra of the CAN layer was about 1 order less than that of the CPN layer. In addition, the CAN surfaces possessed significantly greater wear resistance than the CPN layer at higher loads, and increased the wear resistance of Ti6Al4V alloy by about 2 orders due to the higher loading bearing ability, higher rate of TiN phase, greater surface microhardness and bigger microhardness improving depth. This is because the double glow discharge effect in CAN strengthened the density of the active nitrogen atoms and the nitriding efficiency. Moreover, the ion bombardment effect on the surface of the nitriding titanium alloy was reduced obviously.

The Role of Activated Nitrogen Species on Double-folded Screen Nitriding Process

As clean and energy saving surface hardening technology, plasma nitriding techniques have been evolved with object of higher performance in the last decades. Even though the diffusion of nitrogen inward to steel is occurred at the final step, solid diffusion from surface, energy transition from gas molecule of nitrogen to atomic or an activated state have many different steps depending on the plasma conditions, parameters and the design of each equipment. And this study made comparative on nitrogen sources transfer with conventional DC plasma nitriding and novel nitriding process using plasma diagnosis and metallurgical observation. With different vacuum pressure, gas ratio and new designed electrode (double-folded screen cathode electrode), it showed a different behavior of DC plasma nitriding including the nano-sized nitride on the outer surface of specimen due to nitrogen source of determining plasma species. In this study, plasma species was able to identify with optical emission spectroscopy (OES) studies. From these observations, we could understand better role of ions or neutral nitrogen species, like neutral nitrogen (N), N2+ and NHx radicals in plasma nitriding process with different parameters. And cutting layers of nitride specimen were showed the results due to a different species gas flow ratio or plasma conditions.

Deposition of Nano Sized Titanium Nitride on H11 Tool Steel Using Active Screen Plasma Nitriding Method

Active screen plasma nitriding (ASPN) is an emerging surface engineering technology that offers many advantages over the conventional DC plasma nitriding (DCPN). In this research, plasma nitriding of H11 tool steel, using titanium active screen was investigated. Samples were plasma nitrided at 550oC, for 10 and 20 h with 3 different gas mixtures of H2/N2% = 3, 4, 5. The coating microstructure and phase analysis were investigated using Scanning Electron Microscopy (SEM) and X-ray Diffraction techniques. The dominant phase in the compound layer was TiN. With increasing processing time, the layer thickness was increased. According to SEM results, the sample surface was formed of nano sized and particulate titanium nitride particles. It was shown that with increasing H2% in gas mixtures, better surface quality, golden yellow color, was obtained. Also, the intensity of TiN peaks and layer thickness were increased significantly.

Active screen plasma nitriding – an overview

Extensive researches carried out over the past few years have shown that the novel active screen plasma nitriding (ASPN) technique can be used to treat low alloy steels, stainless steels, tool steels and other steels to achieve identical nitriding effects as the conventional DC plasma nitriding technology. Importantly, the ASPN technique provides the possibilities of treating non-electrical conducting materials such as steel with an oxidised surface and polymeric materials which are unattainable with a conventional DC plasma system. Experimental results presented in this overview further demonstrate that sputtering and deposition play important roles in nitrogen mass transfer in ASPN. In order to achieve a desirable metallurgical response, materials for the active screen and the amount of bias applied to the component have to be considered in applications of active screen plasma processing. The distance between the screen and the component surface also needs to be considered if the components to be treated are placed in a floating potential.

The influence of active screen plasma nitriding parameters on corrosion behavior of a low-alloy steel

This work presents a comparative study of low-alloy steel nitriding for different possible techniques. Active screen plasma nitriding (ASPN) is a successful surface modification method that has many advantages over the conventional DC plasma nitriding (CPN). The corrosion behavior of 30CrNiMo8 low-alloy steel has been examined using anodic polarization tests in 3.5% NaCl solution under varying conditions of ASPN and CPN processes. The process variables included active screen setup parameters, treatment temperature (550 and 580 °C), gas mixture (25/75 and 75/25 of N 2/H 2) and treatment time (5 and 10 h) in 500 Pa pressure. The structure and phases composition of the compound layer was studied by X-ray diffraction (XRD), microhardness tests, optical microscopy and scanning electron microscopy (SEM). It was observed that ASPN treated samples surface enhanced corrosion resistance while the temperature and/or hole size of screen setup increased.

Surface modification of 30CrNiMo8 low-alloy steel by active screen setup and conventional plasma nitriding methods

In this paper, we report on a comparative study of active screen plasma nitriding (ASPN) and conventional dc plasma nitriding (CPN) behavior of 30CrNiMo8 low-alloy steel that has been examined under various process conditions. The process variables included active screen setup parameters (screen and iron plate top lids placed on the screen setup with 8 mm of hole size), treatment temperature (550 and 580 °C), gas mixture (75/25 and 25/75 of N 2/H 2) and treatment time (5 and 10 h) in 500 Pa pressure. The structure and phases composition of the diffusion zone and compound layer were studied by X-ray diffraction (XRD), microhardness tests, light optical microscopy and scanning electron microscopy (SEM). It was observed that treated sample surfaces in both CPN and ASPN methods consist of γ′ and ɛ phases, and while the nitriding time and/or temperature increases, the intensity of ɛ phase in the compound layer will increase for ASPN and decrease for CPN method. Results show that the amount of nitrogen transferred from holes of screen toward the sample surface via sputtering and re-condensation mechanism can be affected due to the hardness and thickness of the layer.

Use of cathodic cage in plasma nitriding

Cylindrical samples of 1020 steel and 316 stainless steel were nitrided under the conditions by conventional dc plasma nitriding (DCPN) and by a new technique denominate cathodic cage plasma nitriding (CCPN). The 1020 and 316 stainless steel samples were treated during 3 h and 5 h, respectively, in 773 K and 360 Pa. The samples were characterized by optical microscopy, X-ray diffraction and microhardness testing. All the samples nitrided by DCPN process presented erosion rings on the surface exposed to the plasma. In comparison, in samples nitrided by CCPN, the erosion rings were completely eliminated, without loss of the mechanical properties in the different phases of existence in the nitrided layer.

Effects of various nitriding parameters on active screen plasma nitriding behavior of a low-alloy steel

Active screen plasma nitriding (ASPN) is a novel surface modification technique that has many capabilities over the conventional DC plasma nitriding (CPN). In this study, 30CrNiMo8 low-alloy steel was active screen plasma-nitrided under various nitriding parameters such as active screen set-up parameters (different screen hole sizes, mesh sheet and plate top lids) and treatment temperature (520, 550 and 580 °C), in the gas mixture of 75% N 2+25% H 2 and chamber pressure of 500 Pa for 5 h. The properties of the nitrided specimens have been assessed by evaluating composition of phases, surface hardness, compound layer thickness and case depth using X-ray diffraction (XRD), microhardness measurements and scanning electron microscopy (SEM). It was found that the screen hole size and top lid type (mesh or plate) play an important role in transition of active species (nitrogen ions and neutrals) toward the sample surface, which in turn can affect the nitrided layer hardness and thickness. Treatment at higher temperature with bigger screen hole size resulted in a thicker compound layer and higher layer hardness. The compound layers developed on the samples treated under different conditions were dual phase consisting of γ′-Fe 4N and ε-Fe 2–3N phases.

Sliding wear properties of active screen plasma nitrided 316 austenitic stainless steel

AISI 316 austenitic stainless steel has been plasma nitrided using the novel active screen (AS) plasma nitriding technique. Wear properties of the AS plasma nitrided and untreated 316 stainless steel have been investigated using a pin-on-disc tribometer with cemented tungsten carbide ball as the counterface. Experimental results have shown that AS plasma nitriding can improve the wear resistance of 316 austenitic stainless steel by more than two orders of magnitude. Wear of untreated 316 stainless steel was severe and characterized by strong adhesion, abrasion and severe plastic deformation. Whilst wear of the AS plasma nitrided 316 stainless steel was mild and dominated by oxidation wear and micro-abrasion. No clear difference, in terms of wear resistance improvement, has been found between the novel AS plasma nitriding and the conventional DC plasma nitriding under the present test conditions.

Active screen plasma nitriding of austenitic stainless steel

Active screen (AS) plasma nitriding is a new surface engineering technique which provides many advantages over conventional dc plasma nitriding. In this study the AS technique has been used to nitride austenitic stainless steel (AISI 316). The morphology, composition and properties of the AS plasma nitrided layers have been characterised and compared with those of the normal dc plasma nitrided layers. AS nitriding can achieve similar hardening effects for austenitic stainless steel when compared to conventional dc nitriding, and does not have the common problems associated with the dc technique such as ‘edging effect’. AS nitriding at a lower temperature of 420° C can also produce a precipitation free layer, the S phase, on a 316 stainless steel surface. The AS nitrided S phase exhibits not only high hardness and wear resistance but also very good corrosion resistance.

A Study of Active Screen Plasma Nitriding

Active screen plasma nitriding (ASPN) is an emerging surface engineering technology that offers many advantages over the conventional dc plasma nitriding (DCPN). In this work, samples of a low alloy steel 722M 24 were AS plasma nitrided with various gas compositions and using different active screen setups. Normal dc plasma nitriding was also performed under similar conditions for comparison. The nitrided samples were characterised with a variety of analytical techniques, including optical microscopy and SEM for morphological and structural examination, XRD for phase identification, and microhardness testing for surface hardness and hardness profile measurements. It has been shown that ASPN can achieve a similar nitriding effect as the normal DCPN but without the common problems associated with dc nitriding such as the arcing surface damage, the edging effect, and the hollow cathode effect. The mechanism of nitrogen mass transfer in AS plasma nitriding has been investigated, and a modified sputtering deposition model has been used to explain the AS plasma nitriding experimental results.