Surface hardening of biomedical Ti–29Nb–13Ta–4.6Zr and Ti–6Al–4V ELI by gas nitriding

The microstructure and hardness near the surface of a biomedical titanium alloy, Ti–29Nb–13Ta–4.6Zr (TNTZ), subjected to gas nitriding at 1023–1223 K was investigated in comparison with the conventional biomedical Ti–6Al–4V ELI (Ti64). After gas nitriding, the microstructure near the specimen surface was observed by optical microscopy, X-ray diffraction (XRD), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). In both alloys, two types of titanium nitrides (TiN and Ti₂N) are formed and the phase is precipitated by gas nitriding. Furthermore, the oxygen impurity in the gas nitriding atmosphere reacts with the titanium nitrides; thus, TiO₂ is formed at the outermost titanium nitride layer. The surface hardening was also evaluated by Vickers hardness measurement. The Vickers hardness near the surface of TNTZ and Ti64 increases significantly by gas nitriding.     

Characteristic sample temperature and pressure during processing of titanium nitride combustion synthesis with liquid nitrogen

Titanium nitride synthesis has been investigated by sustaining a nitriding reaction of titanium powder compacts set in a closed vessel filled with liquid nitrogen. The characteristic features of the present combustion synthesis technology are the accelerating pressure in the vessel following sample heating through its combustion propagation, which results in a specific structure formation of the products. It has been confirmed in the present work that the pressure in the closed vessel was drastically increased (ca. 20 MPa s-1) by propagating the nitriding reaction. The product was shrunk and densified up to 99%. The product was identified as TiN0.87 without any trace of elemental titanium. © 1998 Chapman & Hall     

The effect of different methods to add nitrogen to titanium alloys on the properties of titanium nitride clad layers

In this investigation, titanium nitride (TiN) reinforcements are synthesized in situ on the surface of Ti–6Al–4V substrates with gas tungsten arc welding (GTAW) process by different methods to add nitrogen, nitrogen gas or TiN powder, to titanium alloys. The results showed that if nitrogen gas was added to titanium alloys, the TiN phase would be formed. But if TiN powder was added to titanium alloys, TiN + TiN_x dual phases would be presented. The results of the dry sliding wear test revealed that the wear performance of the Ti–6Al–4V alloy specimen coated with TiN or TiN + TiN_x clad layers were much better than that of the pure Ti–6Al–4V alloy specimen. Furthermore, the evolution of the microstructure during cooling was elucidated and the relationship among the wear behavior of the clad layer, microstructures, and microhardness was determined.     

Influence of the nonstoichiometry of titanium nitride TiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> on the degree of its modification with oxygen

We investigate the influence of the nonstoichiometry (the degree of completeness) of titanium nitride TiN on the degree of its modification with oxygen. The completeness of titanium nitride was controlled by changing the nitriding temperature, nitriding time, and partial nitrogen pressure. It is shown that as the completeness of titanium nitride decreases, which leads to an increase in the intensity of oxynitriding, and as the nitrogen content in the ternary compound TiN _x O_{1 – x} decreases, the level of surface hardening of titanium alloys after oxynitriding diminishes.     

金属钛激光气体氮化层组织及表面特征

采用连续波Nd-YAG激光在氮气环境中对金属钛进行激光气体氮化处理.利用SEM,XRD,XPS研究氮化层的显微组织、表面成分、结构.结果表明:通过激光气体氮化可以在金属钛表面得到表面相对平滑、无裂纹的氮化层;氮化层与基体材料之间为冶金结合.氮化层主要由枝晶状TiN组成,同时有TiNxOy,TiO2及TiC存在,外表面有C,O污染或吸附;TiN枝晶密度由表面沿深度方向下降.     

The effect of laser surface nitriding with a spinning laser beam on the wear resistance of commercial purity titanium

Laser nitriding of commercial purity titanium using various concentrations of helium and nitrogen has been carried out. The surface appearance and microstructure of a treated layer were found to be dependent on the beam power density, interaction time, velocity and concentration of nitrogen. X-ray diffraction analyses have led to the conclusion that the dendrite layer in the resolidified zone of the nitrided specimens consisted mainly of TiN. The surface roughness of specimens after various laser treatments was investigated by SEM and a surface profilemeter. Using optical microscopy, the dendrite TiN and needle-like structure in the melt zone, and the large grain structure in the heat affected zone, were investigated. The surface wear resistance of nitriding CPTi was significantly improved compared to the untreated or laser glazed material, and the wear data were found to correlate with scanning electron microscopy observations. Two layers, having different microstructures, thickness and abrasive wear resistance were identified. Further, 100% overlapping considerably improved the wear resistance of the nitrided specimens.     

Interaction of Si3N4 with titanium at elevated temperatures

In relation to the joining of silicon nitride ceramics to metal, the reaction products and the reaction mechanism between Si3N4 and titanium have been investigated under a nitrogen or an argon atmosphere at temperatures of 823–1573 K. Using Si3N4/titanium powder mixtures, reaction rates were determined by thermogravimetric (TG) analysis, and reaction products were examined by X-ray diffraction. At higher temperatures and on prolonged heating, reaction products were changed in the following orders: TiN2, TiN2+TiN, TiN+TiSi2+Si, TiN+Si and TiN (nitrogen atmosphere) and TiN2+Ti5Si5, TiN2+TiN+Ti5Si3 and TiN+TiSi2+Si (argon). By relating these results to TG measurements, a full understanding of the reaction mechanism between Si3N4 and titanium was acquired. This revised version was published online in November 2006 with corrections to the Cover Date.     

Nitriding titanium under nonisothermal conditions

We consider the process of thermodiffusion saturation of titanium with nitrogen under nonisothermal conditions. We established that a cyclic change in temperature in the course of heating intensifies the process of nitriding. Titanium subjected to cyclic heating has, as a rule, higher mechanical characteristics. We show the influence of the purity of the nitrogen on nitriding titanium under nonisothermal conditions. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 2, pp. 75–78, March–April, 1998.     

Chemical vapour deposition of titanium nitride on plasma nitrided steel

Ferritic and austenitic nitriding by the plasma nitriding technique were investigated for the modification of steel substrates prior to the chemical vapour deposition of titanium nitride at 1273 K. It was confirmed that prenitriding enhances the growth of the titanium nitride layer and it was found that a TiN coating can be formed using substrate derived nitrogen only. Control of porosity, arising during austenitic nitriding, was investigated and it was found that in practice this phenomenon could not be avoided.     

Titanium nitride laser-beam surface-alloying treatment on carbon tool steel

In order to improve the wear resistance of tool steel, a study of TiN surface-alloying treatment on 1% carbon steel by irradiation with a CO₂ laser beam was performed. Argon and nitrogen were used as shielding gases, and their effects on the formation of the surface-alloyed layer were investigated. The effect of cobalt additions to the TiN powder on the hardness of the alloyed layer was also investigated. When argon was used as shielding gas, the depth of the alloyed layer was increased compared with the depth when nitrogen was used as a shielding gas. A portion of the TiN decomposed into titanium in the argon environment, the nitrogen apparently being lost as a gas. The structure of the surface-alloyed layer was composed of a ferritic phase without martensitic structure even at high cooling rates. When this layer was annealed at 1000 ° C for 3 h, part of the titanium precipitated as TiC particles. The hardness of the annealed alloyed layer increased to about 500 Hv. This increase in hardness was accompanied by the appearance of martensite. When nitrogen was used as shielding gas, decomposition of TiN was suppressed and the hardness of the alloyed layer reached 850 Hv. These layers had a martensitic structure. Thus, nitrogen is preferable to argon as a shielding gas if a martensitic structure is desired in this system. When 5% cobalt was added to the TiN powder, the hardness of the alloyed layer increased to 1100 Hv. This increased hardness is caused by stabilization of the martensitic structure caused by an increase in theM _s temperature.     

Microstructure in the weld region in seam welded and resistance welded Zircaloy 4 tubing

The microstructure of tungsten inert gas (TIG)-arc welding and resistance welding in Zircaloy 4 tubing is described in relation to the phases present. Differences in microstructure between the weld zone and base tubing are discussed in terms of the phase transformations that take place during the very different thermal cycles occurring in both processes and a comparison between them is made. A feature of the weld region is the presence of a coarse grained Widmanstätten structure in the TIG welding while a martensitic-type structure or very fine Widmanstätten structure is present in resistance welding.     

氮氩混合比对TA2真空氮化层结构与性能的影响

采用间歇式真空氮化技术对TA2钛合金进行渗氮处理。探究氮氩混合比对合金氮化层结构和性能的影响规律。结果表明:表面渗氮层主要由TiN和TiN0.3相组成,氮氩比越低其有效硬化层越厚,但会降低有效活性N原子的相对含量,不利于渗层的致密性。适当的氮氩混合比能在TA2表面形成氮化物,N原子有效地向纵深扩散,氮化物层与扩散层结合紧密,过渡良好,硬度梯度平缓;腐蚀电位随着氮氩比的增加呈现逐渐上升趋势,从氮氩比为1∶5时的-0.622V提升到氮氩比为5∶1时的-0.549V,腐蚀电流和腐蚀速率则呈现出逐渐降低的趋势。     

Improved Langmuir probe surface coatings for the Cassini satellite

A Langmuir probe will be used on the space research satellite Cassini in order to monitor electron density fluctuations in space plasmas surrounding Saturn. In order to obtain well-defined Langmuir probe characteristics a surface with uniform work function Φ must be used. A graphite coating has been used on earlier earth-bound satellite probes, but for the Cassini satellite to Saturn a hard, wear-resistant coating must be used. In this contribution we report on investigations of titanium nitride (TiN) coatings for this purpose. TiN coatings have been prepared by high temperature nitriding in pure nitrogen gas and by reactive magnetron sputtering on plane titanium and titanium alloy substrates. The resulting coatings have been analysed with laterally resolved, relative Φ measurements, and the tribological properties of the coatings have been investigated by solid particle erosion and scratch testing. In the tests, the graphite coating DAG 213 used on earlier satellite probes was used as a reference material. The results show that the TiN coatings are superior to the graphite coating with regard to both photoelectric properties and solid particle impact resistance. The sputtered coatings exhibit the lowest lateral work function variation, but the nitrided coatings have superior erosion resistance. The results demonstrate that TiN coatings are suitable for space plasma probes.     

Investigations on the effects of plasma-assisted pre-treatment for plasma-assisted chemical vapour deposition TiN coatings on tool steel

Different mixtures of hydrogen, nitrogen and argon were tested for the cleaning and nitriding of cold-working, high chromium tool steel, prior to TiN deposition with the aim of improving adhesion of the TiN layer. It is well known that the condition of the substrate surface and hardening of the substrate by nitriding have a large influence on the adhesion strength of films. Good adhesion was achieved when nitrogen-hydrogen atmosphere with 40%-80% nitrogen (and 20%-60% hydrogen, respectively) was used, the best adhesion quality values were achieved (HF 1-2) with 40% nitrogen. With higher or lower fractions of nitrogen in the pre-treatment gas, adhesion was reduced. Argon addition also had negative effects on the adhesion strength. The microstructure and chemical composition of the near-interface region of the differently pretreated samples were analysed using secondary ion mass spectrometry, X-ray diffraction and light optical microscopy.     

Influence of the parameters of nitriding on the subsurface hardening of VT6 titanium alloy

We study the influence of the parameters of nitriding (temperature, time of holding, and pressure of the active gas) on the formation of nitride coatings based on VT16 titanium alloy. We give recommendations concerning the possibility of combination of the prescribed thermal treatment with the thermochemical (nitriding) treatment of the alloy aimed at guaranteeing the required level of subsurface hardening. It is shown that the decrease in the partial pressure of nitrogen to 1–10 Pa increases the depth of the hardened zone and ensures the required level of subsurface hardening. The procedure of heating in a vacuum (1 mPa) performed prior to the action of nitrogen improves the surface quality of the alloy. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 6, pp. 49–54, November–December, 2007.     

Ion-nitriding behaviour of Fe-Ti alloys in theα-phase region

The ion-nitriding behaviour of four iron alloys containing between 0.11 and 1.48 wt% titanium was investigated in the-phase region to discuss kinetics of the growth of the nitriding layer. The ion-nitriding experiments have been made at 823 K. Two nitriding layers were observed: a thin surface layer which mainly consists of Fe₄N; an internal nitriding layer beneath the surface layer, where the nitride formed was found to be TiN. The growth of the internal nitriding layer is controlled by a diffusion process of nitrogen in the matrix metal. The apparent diffusion coefficient of nitrogen in the nitriding layer, evaluated using the rate equation proposed for internal oxidation, increases linearly with the volume fraction of titanium nitride. Furthermore, by excluding the effect of the titanium nitride from the apparent diffusion coefficient, the diffusion coefficient of nitrogen in-iron was calculated, being in good agreement with that reported so far. In addition, the increase in hardness in the internal nitriding layer has been discussed.     

Vapor phase manufacture of porous TiN deposits on/in porous supports

Particle Precipitation aided Chemical Vapor Deposition (PP-CVD) is a modification of the conventional CVD process. In PP-CVD an aerosol is formed in the gas phase at an elevated temperature, and particles are deposited on a substrate surface. The synthesis of titanium nitride (TiN) using titanium tetrachloride vapor, nitrogen, ammonia, and hydrogen is studied. TiN is formed by a heterogeneous reaction involving a titanium containing species and nitrogen, whereas simultaneously a TiN aerosol is formed by a reaction involving ammonia. Dense columnar microstructures are formed in the absence of a temperature gradient. At low temperature differences between substrate and gas phase only dense microstructures with virtually equiaxed grains are observed. Porous coherent layers are found in experiments, where larger temperature differences are applied. The observed interconnection of the particles originates from a heterogeneous reaction. A further increase in temperature difference between the susceptor and the gas phase only leads to loose powder deposits.     

A dynamic technique for thermophysical measurements at high temperatures in a microgravity environment

Millisecond-resolution dynamic techniques for thermophysical measurements, when utilized in the laboratory, are limited to the study of materials in their solid phase because the specimen becomes geometrically unstable during melting and collapses, due (at least in part) to the influence of gravity. Therefore, a millisecond-resolution dynamic technique is being developed for use in a microgravity environment in order to extend accurate measurements of selected thermophysical properties of electrically conducting refractory materials to temperatures above their melting point. The basic method involves heating the specimen resistively from ambient temperature to temperatures above its melting point in about 1 s by passing an electrical current pulse through it, while simultaneously recording the pertinent experimental quantities. A compact pulse-heating system, suitable for microgravity simulations with NASA's KC-135 aircraft, has been constructed and initial experiments have been performed to study the geometrical stability of rapidly melting specimens. Preliminary results show that rod-shaped specimens can be successfully pulseheated into their liquid phase.Paper presented at the First Workshop on Subsecond Thermophysics, June 20–21, 1988, Gaithersburg, Maryland, U.S.A.Formerly National Bureau of Standards     

Spectroscopic investigations into plasma used for nitriding processes of steel and titanium

Spectroscopic studies of nitriding processes in a conventional d.c. glow discharge have been undertaken. The current work has been devoted to processes and reactions in the gas phase. The emission measurements were made for three nitrogen-hydrogen gas mixtures used for nitriding of steel, and for nitrogen plasma used for nitriding of titanium. Stress was placed in the investigation on the spatially varied emissions of different active species in the space between electrodes to obtain information on the species governing the nitriding of steel and titanium. A dependence of the emission spectra on the discharge gas mixture composition and on the nitriding time was examined for steel. Results of studies of plasma-assisted nitriding processes are discussed in this work.     

Structural stability and mechanochemical activity of titanium nitride prepared by mechanical alloying

Pure titanium nitride (TiN) was synthesized by mechanical alloying (MA). In order to prevent the contamination from the MA vial and atmospheric gas, the MA steel vial was replaced with a titanium vial and atmospheric gas was deoxidized using sponge titanium heated to 623 K. The mechanochemical activity during MA was estimated from the gas purification temperature. The investigation of thermal and pressure stability by thermal treatment and hot isostatic pressing (HIP) revealed that titanium nitride, TiN, was stable on heating to 1173 K under a vacuum, but became unstable under a high pressure, 100 MPa.