Ti Al Si Research Articles

The nanostructured phase transition and thermal stability of superhard f-TiN/h-AlSiN films

The superhard Ti–Al–Si–N films were synthesized by multi-arc ion plating technology and the influence of vacuum annealing on the structures and properties of the films was investigated. Transmission electron microscopy observation confirmed that the as-deposited Ti–Al–Si–N films were consisted of fcc-TiN/hcp-AlSiN multilayers with a period of 8nm. The result also showed that a minute layer of cubic structure was observed in hcp-AlSiN layer at the interface between TiN layers and AlSiN layers, which resulted in an epitaxial growth between TiN layers and AlSiN layers. The annealing experiment of the Ti–Al–Si–N films was performed in vacuum furnace for 2h at temperatures ranging from 700 to 1100°C. With increasing annealing temperatures, no novel phases were observed indicating that the film retained the sharp interfaces. The grains of the film coarsened and showed mixed orientations at 1100°C. The transformation of h-AlSiN into h-AlN and Al-depleted AlSiNx and partial crystalline SiNx was speculated during the annealing process by the XPS and DSC results. The film retained super hardness of above 47GPa even at 1100°C due to the formation of crystalline SiNx and the minute c-(Al, Si)N layer between c-TiN layers and h-AlSiN layers which delayed the transformation of (Al, Si)N from cubic phase to hexagonal phase. The adhesion strength of the film was also discussed and that vacuum annealing could improve the adhesion strength.

Effects of carbon and nitrogen ion implantations on surface and tribological properties of Ti–Al–Si–N coatings

The post-treatment of ion implantation on hard coatings attracts a great deal of attention because of the improvement in coating surface properties. The present research investigates the effect of nitrogen (N), carbon (C) and carbon followed with nitrogen (C+N) ion implantations on the structural and mechanical properties of the Ti–Al–Si–N coatings. Ion implantations were performed at an energy of 50 keV and different doses. The surface properties of the implanted layer were identified by a variety of analytic techniques, such as cross-sectional transmission electron microscopy, energy dispersive spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and nanoindentation measurement. Additionally, the wear performance of the samples was evaluated by a typical ball on disc tribometer in dry conditions. The results showed that the surface properties depended strongly on the implanted species and doses. In addition, a great improvement in the wear was observed on the samples with the post-treatment process of C and C+N ion implantations.

Study of TiAlSiN coatings post-treated with N and C+N ion implantations. Part 2: The tribological analysis

Abstract Ion implantation has found to be an effective approach to modify surface properties of materials. The present research investigates the effect of (1) nitrogen (N), and (2) carbon subsequently with nitrogen (C + N) implantations on the mechanical and tribological properties of the titanium–aluminium–silicon–nitride (Ti–Al–Si–N) coatings. Superhard TiAlSiN coatings produced by magnetron sputtering, of approximately 2.5 μm thickness, were post-treated by implantations of N or C + N at an energy level of 50 keV. The dose range was between 5 × 10 16 and 1 × 10 18 ions cm −2 . After implantation, the tribological performance of the coatings was investigated by a ball-on-disk tribometer against WC–6 wt.%Co ball under dry condition in ambient air. The wear performance of the samples was examined by a variety of characterization techniques, such as secondary electron microscopy (SEM), 3D profilometry, atomic force microscopy (AFM), and micro-Raman. The results showed that the wear performance of the samples depended strongly on the implanted elements and doses. There was slight improvement on the samples implanted with N whereas significant improvement was found on the C + N implantations. Particularly, the friction coefficient of the sample with 5 × 10 17 C + cm −2 and 5 × 10 17 N + cm −2 could reach 0.1. In addition, the specific wear rate of the sample was extremely low (0.85 × 10 −7 mm 3 /Nm), which was nearly two orders of magnitude below that of the un-implanted coating. The speculation of the mechanical and tribological analyses of the samples indicates that the improvement of the N implanted and C + N implanted TiAlSiN samples could be due to a combined effect of improved hardness, plus enhanced adhesive and cohesive strength. In addition, the improved performance of the C + N implanted samples could be explained by the formation of lubricating implanted-layer, which existed mostly in sp 2 C–C and C–N forms. The formation of such implanted layer could lead to a change of wear mode from strong abrasive wear to mostly adhesive wear, and result in a drop of friction coefficient and wear rate.

Characterization and properties Ti–Al–Si–N nanocomposite coatings prepared by middle frequency magnetron sputtering

TiN-containing amorphous Ti–Al–Si–N (nc–TiN/a–Si 3N 4 or a–AlN) nanocomposite coatings were deposited by using a modified closed field twin unbalanced magnetron sputtering system which is arc assisted and consists of two circles of targets, at a substrate temperature of 300 °C. XRD, XPS and High-resolution TEM experiments showed that the coatings contain TiN nanocrystals embedded in the amorphous Si 3N 4 or AlN matrix. The coatings exhibit good mechanical properties that are greatly influenced by the Si contents. The hardness of the Ti–Al–Si–N coatings deposited at Si targets currents of 5, 8, 10, and 12 A were 45, 47, 54 and 46 GPa, respectively. The high hardness of the deposited Ti–Al–Si–N coatings may be own to the plastic distortion and dislocation blocking by the nanocrystalline structure. On the other hand, the friction coefficient decreases monotonously with increasing Si contents. This result would be caused by tribo-chemical reactions, which often take place in many ceramics, e.g. Si 3N 4 reacts with H 2O to produce SiO 2 or Si(OH) 2 tribolay-layer.

Oxidation Resistance of Ti–Si–N and Ti–Al–Si–N Films Deposited by Reactive Sputtering Using Alloy Targets

Ti–Si–N and Ti–Al–Si–N films, which possess high hardness due to the formation of a nanocomposite structure in the films, were deposited by reactive magnetron sputtering using alloy targets and then postannealed in air at temperatures ranging from 300 to 800 °C. The hardness of both the films decreased significantly as postannealing temperature increased. However, the hardness of Ti–Al–Si–N films postannealed up to 500 °C remained at more than 30 GPa, which was significantly higher than that of the Ti–Si–N films after the post annealings. Electron probe microanalyses and X-ray photoelectron spectroscopy revealed that Al2O3 phases were formed in the postannealed Ti–Al–Si–N films. Transmission electron microscopy with energy-dispersive X-ray analysis showed that the Al2O3 layer of the postannealed Ti–Al–Si–N films was formed 40 nm below the surface, whereas the depth of the TiO2–SiO2 layer of the postannealed Ti–Si–N films was 100 nm from the surface. These results indicate that Al2O3 phases existed at the surface of the Ti–Al–Si–N films and prevented the oxidation of the interior of the films during postannealing at high temperatures in air.

Oxidation Resistance of Ti–Si–N and Ti–Al–Si–N Films Deposited by Reactive Sputtering Using Alloy Targets

Oxidation Resistance of Ti–Si–N and Ti–Al–Si–N Films Deposited by Reactive Sputtering Using Alloy Targets

Comparison of titanium silicide and carbide reinforced in situ synthesized TiAl composites and their mechanical properties

This study explored a synthesizing route involving in situ development of reinforcements of titanium silicides in a series of TiAl-based matrices. The main features of this processing route are: (1) Incorporating a small quantity of mechanically alloyed Ti–Al–Si and Ti–Al–Si–C powders, referred to as precursors, into Ti–Al–X (X stands for Cr, Mn, Nb, or their combination) powder mixtures that act as matrices; and (2) Hot isostatic pressing (HIPing) of the cold compacted mixture at a temperature of 1100 °C for 4 h. A series of composites based on different Ti–Al–X matrix were synthesized. The structural evolution in these composites was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical properties and compression tests at room and elevated temperatures (600 °C and 800 °C) have been investigated. In addition, the influence of alloying elements in the matrix has been evaluated from their stress–strain responses.

Ti–Al–Si–C–N Hard Coatings Synthesized by Hybrid Arc-Enhanced Magnetron Sputtering

Abstract Ti–Al–Si–C–N coatings are deposited by hybrid arc-enhanced magnetron sputtering and characterized by various micro and macro-tools. X-ray diffraction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy reveal that the coatings are probably nanocomposites consisting of nanocrystallites and amorphous phases. They are generally in the form of nc-(Ti,Al)(C,N)/a-SiN/a-C, depending on the composition of the coatings. With increasing Al concentrations, the X-ray diffraction peaks shift to a lower angle and the measured hardness diminishes, implying a reduced contribution of the self-organized stable nanostructure. The dry friction coefficients of the Ti–Al–Si–C–N coatings are about 0.3, which is lower than that of conventional Ti–Si–N coatings. These coatings can find potential applications requiring heavy contact loading subject to high temperature.

Effect of reactive sintering conditions on microstructure of in situ titanium aluminide and silicide composites

In this work, the dependence of microstructure of in situ titanium aluminide and silicide composites on chemical composition and conditions of reactive sintering was studied. It was found that the rate of reactive sintering process increases with growing silicon content. In addition, heating rate strongly affects the structure of Ti–Al–Si alloys with lower content of silicon represented by TiAl23Si10 alloy. The structure consisting of homogeneously distributed Ti5Si3 particles in TiAl matrix can be obtained by rapid heating over 250 K min−1 in this material. TiAl15Si15 alloy forms the desired two phase TiAl–Ti5Si3 structure when a heating rate of at least 10 K min−1 and temperature over 750°C are applied. Replacement of aluminium and silicon powders by Al–Si alloy positively affects the reactive sintering process. The structure of these materials can be refined by milling of Al–Si master alloy powder to the <100 μm fraction, reducing the size of Ti5Si3 particles and porosity significantly.

Machining performance of Ti–Al–Si–N coated inserts

Ti–Al–Si–N quaternary coating has recently been developed for industrial applications due to its excellent machining performance. Here, we present a comparative research on Ti–Al–N single layer, Ti–Al–Si–N single layer, TiAlN–TiAlSiN bilayer and TiAlN/TiAlSiN multilayer coatings deposited onto cemented carbide substrates by cathodic arc evaporation. The incorporation of Si into the Ti–Al–N coating results in an increase in hardness and thermal stability due to the formation of nanocomposite nc-TiAlN/a-Si 3N 4, and thereby causes an improved performance during continuous cutting. However, the lower toughness and adhesive strength with a substrate reduce its cutting-life during milling. Further optimization of Ti–Al–Si–N coated inserts during milling can be obtained by a structure adjustment from the nanocomposite into TiAlN–TiAlSiN bilayer and TiAlN/TiAlSiN multilayer coatings, which causes an increase to 156% and 172% for the life-time of Ti–Al–Si–N coated inserts, respectively. Our results indicate that the machining performance of coatings containing Si in both continuous cutting and milling can be optimized by the structure design of the TiAlN/TiAlSiN multilayer, where the coating sustains a high hardness of the Ti–Al–Si–N coating combined with a good cohesive strength with the substrate similar to the Ti–Al–N coating.

High Temperature Stability of Multicomponent TiAlSiN and CrAlSiN Coatings

The high temperature oxidation behavior of TiAlSiN and CrAlSiN coatings was studied. These coatings were deposited on silicon substrates by using a cathodic-arc deposition system with lateral rotating arc cathodes. Titanium, chromium and Al88Si12 cathodes were used for the deposition of TiAlSiN and CrAlSiN coatings. All the deposited Ti(0.49)Al(0.44)Si(0.07)N, Ti(0.41)Al(0.51)Si(0.08)N and Cr(0.50)Al(0.440Si(0.06)N coatings showed B1-NaCl crystal structure and possessed nano-grain sizes of 6-8 nm. For the high temperature oxidation test, the coated samples were annealed at 900 degrees C in air for 2 hours. The Ti(0.41)Al(0.51)Si(0.08)N with higher Al and Si contents possessed lower oxidation rate than that of Ti(0.49)Al(0.44)Si(0.07)N. The oxide layer formed on the Ti(0.49)Al(0.44)Si(0.07)N coatings consisted of large TiO2 and TiAlSiN grains at the oxide-coating interface, followed by a layer of Al2O3 in the near-surface region. The oxidation rate of the Cr(0.50)Al(0.44)Si(0.06)N coated sample was much lower than that of the Ti(0.49)Al(0.44)Si(0.07)N and Ti(0.41)Al(0.51)Si(0.08)N. The dense Al2O3 with amorphous top layer at the oxide-coating interface retarded the diffusion of oxygen into the Cr(0.50)Al(0.44)Si(0.06)N. The deposited Cr(0.50)Al(0.44)Si(0.06)N showed a high temperature performance superior to those of the Ti(0.49)Al(0.44)Si(0.07)N and Ti(0.41)Al(0.51)Si(0.08)N.

High-temperature behaviour of Ti–Al–Si alloys produced by reactive sintering

Alloys formed by Ti–Al intermediary phases have low density and very good high-temperature oxidation resistance. Further improvement of high-temperature behaviour can be achieved by a suitable addition of silicon. In this work, Ti–Al–Si alloys containing 20 wt.% of aluminium and 10–20 wt.% of silicon produced by a reactive sintering technology were tested. Isothermal oxidation tests were carried out at 1000 °C in air. The oxidation rate was determined from weight gains caused by oxide formation. Microstructure and phase composition of the oxide layers were studied. Microstructure and hardness changes after various annealing durations were described.

Microstructural Characterization of Silicon Added Titanium Aluminide

Titanium aluminides intermetallic compounds have received great attention during the past decade, since they have the potential, in aircraft and automotive engines, to replace the high density Ni-base superalloys However, these intermetallics possess poor oxidation properties at high temperatures. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. In the present study, a number of cast Ti–Al–Si alloys were investigated in relation to transient oxide formation in air at 1300°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques. The TiAl–Si alloys appear to form Al Ti and Si oxides. However, the formation of silicon oxide at the interface of base metal and scale slows down the oxidation rate significantly.

Reaction pathway and phase transitions in Al–Ti–Si system during differential thermal analysis

The present paper mainly studied the phase formation and reaction pathway of the Al–Ti–Si system in detail by thermal analysis combined with XRD and SEM observations. The phase formation sequence in Al–Ti–Si system from starting mixtures to final products with increasing temperature can be described as following: Al (l) + Ti (s) + Si (s) → (Al–Si) (l) + Ti (s) + Si (s) → Ti(Al,Si) 3(s) + Si (s)Ti 5(Si,Al) 3 + Al (l). More importantly, the solubility of Si in Ti(Al,Si) 3 decreased gradually while that of Al in Ti 5(Si,Al) 3 increased with temperature increasing, suggesting the transportation of Si atoms from intermediate aluminides Ti(Al,Si) 3 to final stable silicides Ti 5(Si,Al) 3 and hence further confirming the formation of Ti 5(Si,Al) 3 at the expense of Ti(Al,Si) 3.

Effect of Si addition on microstructure and mechanical properties of Ti–Al–N coating

Abstract Ti–Al–N coatings are widely used to prevent the untimely consumption of cutting tools exposed to wear. Increasing requirements on high speed and dry cutting application open up new demands on the quality of wear-protective quaternary or multinary Ti–Al–N based coating materials. Here, we investigated the microstructure and mechanical properties of Ti–Al–N and Ti–Al–Si–N coatings deposited on cemented carbide by cathodic arc evaporation. The formation of nanocomposite nc-TiAlN/a-Si 3 N 4 structure by incorporation of Si into Ti–Al–N coating causes a significant increase on hardness from ∼ 35.7 GPa of Ti–Al–N to ∼ 42.4 GPa of Ti–Al–Si–N. Both coatings behave age-hardening during thermal annealing, however Ti–Al–Si–N coating reveal better thermal stability. Therefore, the improved cutting performance of Ti–Al–Si–N coated inserts is obtained compared to Ti–Al–N coated inserts.

Properties of (Ti,Nb)Al–(Ti,Nb)5Si3 eutectic composite

Abstract Ti–40Al–5Si and Ti–39Al–5Si–2Nb (in at.%) alloys were studied as prospective high-temperature structural composites consisting of γ-(Ti,Nb)Al + α 2 -(Ti,Nb) 3 Al matrix and Ti 5 Si 3 reinforcement. The alloys were prepared by arc melting under helium. Oxidation resistance was studied at 900 °C in air. Thermal stability of alloys was investigated by measuring room temperature hardness and compressive strength after long-term annealing at 900 °C. To prepare oriented composites, directional crystallization at rates of 5–115 mm/h was carried out by the floating zone technique. It was observed that the addition of 2% Nb to the Ti–40Al–5Si alloy does not modify eutectic structure. Niobium is almost uniformly distributed in all present phases. Both alloys show excellent oxidation resistance at 900 °C in air. The Nb-addition causes significant improvement of oxidation resistance due to the doping effect and increase of Al activity in the scales. Room temperature hardness and compressive strength of both as-cast alloys are similar – about 500 HV and 1600 MPa, respectively. Room temperature mechanical properties do not reduce significantly after 300 h annealing at 900 °C, due to a high morphological stability of eutectic silicides. Directionally solidified alloys consist of columnar Ti–Al grains elongated in crystallization direction and silicides. Niobium refines both Ti–Al grains and Ti 5 Si 3 silicides. As a consequence, orientation and elongation of silicides in the Nb-containing alloy are reduced. In the Ti–Al–Si alloy directionally crystallized at 5–115 mm/h, the silicide interparticle spacing λ (in mm) is related to the crystallization rate R (in mm/h) by a following expression: λ 1.33 · R = 0.32 . In the Nb-containing alloy, silicide interparticle spacing does not depend on the crystallization rate.

Effect of high-energy carbon ion implantation on Ti–Al–Si–N coatings by metal vapour vacuum arc

Due to the combination of high hardness, good thermal stability and oxidation resistance, Ti–Al–Si–N coatings have become an excellent candidate for use as high speed machining coatings. Nowadays ion implantation has been employed to alter the near-surface structure and properties of semiconductors, metals and metal alloys without the loss of bulk properties. The present research investigated the effect of carbon ion implantation on the structural, mechanical and tribological properties of the Ti–Al–Si–N coatings prepared by reactive unbalanced magnetron sputtering. Carbon implantation was carried out by metal vapour vacuum arc ion source (MEVVA) with different carbon ion doses. After implantation, the samples were investigated systematically by means of the Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), cross-sectional transmission electron microscopy (TEM), X-ray photo-electron spectroscopy (XPS), surface profilometry (PGI), nano-indentation measurement and ball-on-disk test etc. The results showed that the implantation depth, chemical composition, microstructure and nanohardness depended strongly on the implanted carbon doses. In addition, a great improvement on the tribological performance was observed with the post-treatment process of carbon ion implantation.

High temperature oxidation resistance of multicomponent Cr–Ti–Al–Si–N coatings

Nanosrystalline CrAlSiN and CrTiAlSiN coatings were deposited by using a cathodic-arc deposition system with lateral rotating arc cathodes. Titanium, chromium and Al 89Si 11 cathodes were used for the deposition of CrAlSiN and CrTiAlSiN coatings. For the high temperature oxidation test, the coated samples were annealed at 900 °C in air for 2 h. The oxidation characteristics of the deposited coatings were also studied by thermal-gravimetric analysis (TGA). In this study, chemical composition and bonding states of the deposited and annealed CrAlSiN and CrTiAlSiN coatings were evaluated by X-ray photoelectron spectroscopy (XPS). All the deposited Cr 0.36Al 0.57Si 0.07N and Cr 0.40Ti 0.22Al 0.36Si 0.02N coatings showed B1-NaCl crystal structure and possessed nano-grain sizes of 8–12 nm. The oxide scale formed on the Cr 0.40Ti 0.22Al 0.36Si 0.02N coatings consisted of major TiO 2 at the surface region, followed by a mixed layer of Al 2O 3 and Cr 2O 3 phases. TiO 2 grew at an accelerated rate on to the aluminum and chromium oxide layers after oxidation at 900 °C. The oxidation rate of the Cr 0.36Al 0.57Si 0.07N coated sample was much lower than that of Cr 0.40Ti 0.22Al 0.36Si 0.02N. A dense protective layer, mainly consisted of Al 2O 3 and Cr 2O 3, retarded the diffusion of oxygen into Cr 0.36Al 0.57Si 0.07N. It indicated that Cr 0.36Al 0.57Si 0.07N with higher Al, and Si contents possessed superior oxidation resistance than Cr 0.40Ti 0.22Al 0.36Si 0.02N.

Structural evolution of Ti–Al–Si–N nanocomposite coatings

Ti–Si–Al–N films were prepared by rf reactive magnetron sputtering, in static and rotation modes, using a wide range of different deposition conditions, which created conditions to obtain Ti–Al–Si–N coatings with different structural arrangements. Films prepared below a critical nitrogen flow, under conditions out of thermodynamic equilibrium, revealed a preferential growth of an fcc (Ti,Al,Si)N x compound with a small N deficiency. With nitrogen flow above that critical value, the reduction of the lattice parameter was no longer detected. However, a thermal annealing showed that a complete thermodynamically driven segregation of the TiN and Si 3N 4 phases was not yet obtained. The segregation upon annealing induced a self-hardening and showed a multiphase system, where the crystalline TiN, (Ti,Al)N and (Ti,Al,Si)N x phases were identified by X-ray diffraction. This behavior is due to the de-mixing of the solid solution associated to a small N deficiency.

Studies on wear behavior of nano-intermetallic reinforced Al-base amorphous/nanocrystalline matrix in situ composite

Resistance to wear is an important factor in design and selection of structural components in relative motion against a mating surface. The present work deals with studies on fretting wear behavior of in situ nano-Al 3Ti reinforced Al–Ti–Si amorphous/nanocrystalline matrix composite, processed by high pressure (8 GPa) sintering at room temperature, 350, 400 or 450 °C. The wear experiments were carried out in gross slip fretting regime to investigate the performance of this composite against Al 2O 3 at ambient temperature (22–25 °C) and humidity (50–55%). The highest resistance to fretting wear has been observed in the composites sintered at 400 °C. The fretting wear involves oxidation of Al 3Ti particles in the composite. A continuous, smooth and protective tribolayer is formed on the worn surface of the composite sintered at 400 °C, while fragmentation and spallation leads to a rougher surface and greater wear in the composite sintered at 450 °C.