Papers by Keyword: Plasma Nitriding

Abstract: Duplex treatments are surface modification techniques used to enhance the surface properties of steels. In this study, hard chromium electroplated AISI 4340 steel was plasma nitrided in pure nitrogen gas at 600°C for 90 min. The structural and morphological properties were analysed by an X-ray diffractometer and a scanning electron microscope (SEM), respectively. The hardness profile was obtained by a multicycle nanoindentation test and the tribological properties by a ball-on-disk test at room temperature. The results showed that the duplex layer was composed mainly of Cr₂N and it had a rough surface morphology with a network of micro-cracks. The highest values of hardness were registered in the nitrogen diffusion layer beneath the outer porous thin layer. The friction coefficient and the specific wear rate after plasma nitriding were reduced by 3 and 5 factors, respectively.

Abstract: Ti alloys have been intensely used for human implants due to its excellent characteristics, like bio-inertness, low density, and corrosion resistance. However, some alloying elements were found to be toxic for the human body, which restricts the use of some alloys. Furthermore, there are two additional and essential aspects to be considered. The first relates to the young modulus that, despite being lower than other alloys commonly used for this purpose, it is still far over from the human bone modulus. Such high modulus can result in the stress shield phenomena and the consequent implant losing. The second aspect relates to the fact that bio-inertness does not guarantee a complete tissue integration to the implant and, consequently, the expected implant performance. In this context, new low modulus b-Ti alloys containing nontoxic elements have been developed in recent years, and several surface modification processes have been proposed to promote better implant/tissue integration.In the present work, the new b-type Ti-Mo-Zr-Fe alloy has been submitted to a plasma enhanced chemical vapor deposition (PECVD) process in order to form a superficial titanium nitride layer, aiming to produce a satisfactory substrate for the tissue cells growing. In a first step, microstructural characterization and corrosion performance of the modified alloy surface has been evaluated by Electrochemical Impedance Spectrometry and Potentiodynamic testing, and the results compared to the unmodified alloy. It was found that during the plasma nitriding process, that runs at 550°C for 1h, the metastable b microstructure is partially converted into a’ and possibly a” phases, which can impact the young modulus. The 500nm thick TiN layer formed over the alloy surface improved the corrosion behavior of the alloy. These results encourage the continuity of the research, with the future in vitro bio-activity testing of the nitrided surface.

Abstract: TiN coatings are widely used in different applications for extending the lifetime of components due to their high hardness and good wear resistance. However, it is not convenient to deposit them on soft stainless steels. In this work, the wear and corrosion behavior of commercial TiN coatings deposited by Arc-PVD on nitrided and non-nitrided martensitic stainless steel was studied. Two different nitriding conditions were used, one at high temperature (HTN) and the other at low temperature (LTN). Nanohardness and microhardness were measured. The microstructure was characterized by OM, SEM, XRD and XPS. Pin on disk and erosion tests were carried out in order to evaluate their wear resistance. The corrosion behavior was analyzed in salt spray fog and electrochemical tests in NaCl solution and the adhesion was measured in Scratch and Rockwell C Indentation tests.The coating thickness was about 1.5 µm and its hardness of 34 GPa. The nitrided layers were 13 µm and 17 µm thick for LTN and HTN, the hardness was approximately 12 GPa for both nitrided samples. The nitrided layer improved TiN coating adhesion in the Scratch tests. The wear loss volume was similar for both duplex and only coated samples in pin on disk tests. Nevertheless, wear resistance was not good for the LTN or HTN + TiN coating system in the erosion tests. Regarding corrosion behavior, the coatings showed poor corrosion resistance and this could be related to the presence of porous defects, which allow the solution to reach and attack the substrate, thus producing coating detachment around the pits.

Abstract: Plasma nitriding has attracted much interest to improve the hardness of aluminium alloys. However, the contradictive properties can be produced on the metal surface due to the saturated condition of the diffused nitrogen atom in the metal surface layer. The objective of this work was to investigate the effect of nitriding time and gas pressure to improve the hardness of aluminium using plasma nitriding. The nitriding processes were conducted in a DC glow discharge with nitrogen gas flowing inside the vacuum chamber. Firstly, the sample was nitrided using a fixed gas pressure of 1.2 mbar with the varied nitriding times of 3, 4, 5 and 6 hours. The optimum time producing the highest hardness of the surface was then used in the next nitriding process with varied gas pressure of 1.2, 1.4, 1.6 and 1.8 mbar (1 bar = 10⁵ Pa). The optimum gas pressure producing the highest hardness was then used again in the last nitriding process using varied nitriding time of 3, 4, 5 and 6 hours. The result showed that the highest hardness was achieved using the gas pressure and nitriding time of 1.6 mbar and 4 hours, respectively. The formed AlN phase on the aluminium surface was identified by XRD, whereas the surface morphology was observed by SEM image. Compared to the untreated sample, the hardness of the treated samples was significantly high.

Abstract: The dry progressive stamping was strongly required to make mass production of clothing parts and beverage cans. The duplex coating was one of the most reliable means to protect the dies and punches from wear and friction and to prolong their life time. In this coating, the die and punch was first surface-treated to have sufficient hardness in compatible to the hard ceramic coatings. In the present study, the low temperature plasma nitriding at 673 K was employed to harden the six kinds of punches and dies for progressive stamping of copper alloyed fucks. The micro-structure and nitrogen mapping were investigated by SEM with EDS to demonstrate that the hardening took place by nitrogen super saturation into SKD11 matrix without nitride precipitations. These nitrogen super-saturated punches and dies were fix into the progressive die set for dry stamping. No significant wear of tools as well as reduction of stamping loads even after a million shots proved that the low temperature plasma nitriding should be suitable to make hardening of dry stamping die substrates even without use of hard ceramic coatings.

Abstract: The influence of plasma nitriding of a maraging 300 steel on mechanical properties at high temperature has been studied. Samples were tensile tested at 600°C in four conditions: solution treated (MAR-S), solution treated and aged (MAR-SA), solution treated and plasma nitrited (MAR-SP) and solution treated, aged and plasma nitrited (MAR-SAP). In the same sequence, the yield strength and ultimate tensile strength increased slightly respectively from 1073 to 1189 MPa and 1174 to 1301 MPa, an increase of about 10% due to plasma nitriding. All the samples presented similar values of elongation, around 18%, but the cross section area reduction decreased significantly by plasma nitriding from ~70% for MAR-S and MAR-SA to ~45% for MAR-SP and MAR-SAP, that is an decrease of 36% in average. This decrease is attributed to brittle fracture nucleated at 50 μm thick iron nitride layer. The inner fracture surface of the tensile tested specimens was predominantly ductile presenting characteristic microcavities.

Abstract: The low hardness and poor tribological performance of titanium alloys restrict their wide applications in automotive fields. Nitriding is widely used to improve tribological properties, wear resistance, and corrosion resistance of steel and titanium alloys. Plasma nitriding is becoming increasingly popular because of its high nitrogen potential, short treatment time, and low environmental impact. Recently, considerable interest has been devoted to alternative nitriding methods such as active screen plasma nitriding (ASPN). In this study, a Ti-6Al-4V titanium alloy was nitrided by ASPN using a titanium double screen in order to investigate the effect of applying the double screen on the microstructure of the nitriding layer. The Ti-6Al-4V sample was placed on the sample stage in a cathodic potential. A titanium double screen was mounted on the cathodic stage around the sample stage. The sample was treated for 1-25 hours at 600oC under 200 Pa in 75% N₂ + 25% H₂ atmosphere. After nitriding, glow discharge optical emission spectroscopy (GD-OES) revealed that the thickness of the nitriding layer composed of TiN tended to increase with increasing the nitriding time. The Vickers microhardness of the sample surface nitrided for 25 hours reached approximately 1300 HV. Ball-on-disk wear test revealed that a wear loss of nitrided sample considerably decreased than that of untreated sample.

Abstract: Fatigue characteristics of steel 41CrAlMo7-10 and topography analysis testing methodology of fish eye were studied. Both sets of samples were heat-treated at the same conditions and subsequently were plasma nitrided. The mixture ratio of these main gases, forming the atmosphere during the plasma nitriding was inverted for both sets of samples. Process parameters such as the duration, the applied voltage and the pulse length were identical for all samples. Following test after heat treatment and surface layer formation by plasma nitriding process for both sets of samples was the rotating bending fatigue testing in the machine R.R.Moore L2568 of Instron Co. The fatigue tests were carried out in accordance with the CSN 42 0363 Fatigue Testing of Metals, Methodology of Testing. Then, tests were evaluated numerically and graphically using Wöhler`s (S-N) curve in a semi-logarithmic form. The testing was stopped after reach the fracture of tested sample or after 10⁷ cycles. The surface topography of fatigue cracks were evaluated on Talysurf CCI. The topography analysis was mainly focused on the initial part of fatigue crack called the fish-eye. The topography analysis was performed only on one selected sample of each series.

Abstract: This article deals with properties of plasma nitrided layers in depth. The redistribution of alloy elements and C-N in depth of nitrided layers was analysed. The plasma nitriding process was used for creation of nitrided layers [1]. The nitrided layers were applied to 32CrMoV12-10 steel used for stressed machine parts. Samples were subsequently evaluated by metallographic, OES, hardness and microhardness methods. The results of measurement showed connection between chemical composition of alloying elements after chemical-heat treated process and universal hardness, microhardness.

Abstract: The article deals with an alteration of mechanical properties of plasma nitrided steel. Experimental work was focused on evaluation of influence of plasma nitriding process on the notch toughness of steel. Experiments were realized on V-notch samples of size 10x10x55 mm (according to ISO 148-1 standard) from 14NiCr14 steel. The 14NiCr14 steel was heat treated prior plasma nitriding. The heat treatment consisted of hardening at 850 °C and tempering on elimination of internal stress at 200 °C. Heat treated experimental samples were plasma nitrided at 450 °C, 500 °C and 550 °C. The notch toughness tests of steel were carried out using the instrumental Charpy hammer at test temperatures-40 °C, +21 °C and +70 °C. The notch toughness of plasma nitrided samples achieved nearly identical values as heat treated samples thus plasma nitriding did not reduce the values of notch toughness. The increase in temperature of the plasma nitriding process even led to achievement of higher values of notch toughness compared with heat treatment specimens namely at all test temperatures.