Ti-rich Layer Research Articles

Surface engineering of hematite nanorods by 2D Ti3C2-MXene: Suppressing the electron-hole recombination for enhanced photoelectrochemical performance

Photoelectrochemical (PEC) water splitting is a promising approach to improve solar energy conversion, but still suffers from poor efficiency due to intrinsic high charge recombination and low conductivity of semiconductor photocatalysts. In this work, we applied Ti3C2-MXene to construct hematite/MXene nanorods (α-Fe2O3/MXene NRs) with enhanced PEC performance. The as-formed MXene-derived Ti-rich layer could not only serve as a passivation layer to significantly suppress surface electron-hole recombination, but also act as Ti source to promote bulk electron transfer. The optimum α-Fe2O3/MXene5/1 NRs resulted in a 7 times PEC enhancement compared with pristine α-Fe2O3 NRs while maintaining high stability. Mechanism studies by in-situ ultrafast transient absorbance spectra (TAS) directly proved that the duration of the photo-induced holes for optimum α-Fe2O3/MXene NRs was 10 times longer than pristine α-Fe2O3 NRs, proving notably enhanced surface charge separation. This study gives a clue to construct series of MXene based anode materials with promoted performance.

Effect of annealing on the microstructure and mechanical properties of Ti0.17Al0.83N coating prepared by low pressure chemical vapor deposition

The face-centered-cubic (fcc) Ti 0.17 Al 0.83 N coating was deposited on cemented carbide insert by low pressure chemical vapor deposition (LPCVD) method. The effects of annealing temperature and atmosphere on the surface and fracture morphology, phase structure, residual stress, and hardness of the Ti 0.17 Al 0.83 N coating were studied. TEM analysis indicated that the as-deposited Ti 0.17 Al 0.83 N coating was self-organized with a periodically alternating Al-rich (9 nm) and Ti-rich (2 nm) nanolamellae. The width of the Al-rich (12 nm) and Ti-rich (3 nm) nanolamellae was increased after annealing in vacuum at 900 °C for 1 h. With the rise of vacuum annealing temperature from 700 to 1200°C, the coating hardness was increased and then decreased while the compressive residual stress was increasing continuously. A maximum hardness of 46.5 GPa was obtained after annealing in vacuum at 900 °C for 1 h. The coating grains cracked with the compressive residual stress of −1.55 GPa after annealing in vacuum at 1100 °C for 1 h. The surface morphology of the coating was basically unchanged after annealing in air at 900 °C for 1 h, while it was completely oxidized with the formation of Al 2 O 3 on the surface after annealing at 1000 °C. • Residual stress of CVD Ti 0.17 Al 0.83 N coating after vacuum annealing was analyzed. • The fraction of Al in Al- and Ti-rich nanolamellae layers are ~0.9 and ~0.6, respectively. • Coating structure before and after vacuum annealing was investigated by TEM analysis. • CVD Ti 0.17 Al 0.83 N hardness reached 46.5 GPa after vacuum annealing at 900 °C for 1 h.

Characterization of microstructure and surface oxide of Ti1.2Fe hydrogen storage alloy

In this study, we investigated the microstructures, hydrogen absorption kinetics, and oxide layers of TiFe and Ti1.2Fe hydrogen storage alloys. Whereas the TiFe alloy has a single phase, the Ti1.2Fe alloy is composed of three phases: TiFe, Ti2Fe, and Ti4Fe. Under no thermal activation process, the TiFe alloy does not absorb hydrogen, though the Ti1.2Fe alloy starts to absorb hydrogen after 4 min of incubation time. From the XPS results, it is revealed that the Ti concentration in the oxide layer on the Ti4Fe phase is higher than that on the TiFe phase, indicating that the Ti concentration in the oxide layer would be important in improving hydrogen absorption kinetics. Based on these results, the hydrogen absorption kinetics could be improved by adjusting composition, enabling the formation of a Ti-rich oxide layer.

Influence of Growth Defects on the Oxidation Resistance of Sputter-Deposited TiAlN Hard Coatings

This paper reports the results of an investigation of the oxidation of a sputter-deposited TiAlN hard coating in air at temperatures of 800 and 850 °C for times ranging from 15 min to 2 h. The study is focused on the role of growth defects in the oxidation process. The mechanism of oxidation at the site of the defect was studied on cross-sections made by the consecutive sectioning of oxidized coatings with the FIB technique. We found that in the early stage of oxidation, the locally intense oxidation always starts at such defects. Although the growth defects reduce the oxidation resistance of the coating locally, we believe that they do not have a decisive influence on the global oxidation resistance of the coating. There are several reasons for this. The first is that the surface area covered by growth defects is relatively low (less than 1%). Secondly, the coating is permeable only at those defects that extend through the entire coating thickness. Thirdly, the permeability at the rim of some defects strongly depends on the density of pores at the rim of defects and how open they are. The size and density of such pores depend on the shape and size of topographical irregularities on the substrate surface (e.g., seeds, pits), which are responsible for the formation of growth defects. We also found that oxidation of the TiAlN coating is accelerated by oxygen and titanium diffusion through the pores formed by crystal grain growth in the outer alumina overlayer. Such pores are formed due to the compressive stresses in the Ti-rich oxide layer, which are caused by the large difference in molar volumes between the oxide and nitride phases.

The nucleation sequence of α-Al on TiB2 particles in Al-Cu alloys

The refinement of Al alloys by TiB2 has been extensively investigated for decades, both in industry and academia. In order to achieve higher grain refinement potency, it is known that other solutes must be added alongside TiB2, thus tailoring the heterogeneous nucleation at the interface between the TiB2 particles and the Al matrix. Here, we report results from an atomic-scale experimental investigation of the heterogeneous nucleation interface of TiB2 in Al-Cu based alloys as well as in an Al-5Ti-1B grain refiner employed in these Al-Cu based alloys. We focus on the effect of segregation of the main solute elements (Ti, Cu) to the interface of TiB2 and attempt to disentangle this effect from other factors affecting the nucleation and growth. Significant Ti segregation in the form of a Ti-rich layer, identified as an Al3Ti two-dimensional compound, was unambiguously observed on the basal plane of TiB2 particles in the Al-5Ti-1B grain refiner, in agreement with prior literature. In the Al-Cu alloy system, prevalent Ti segregation was also observed on the basal plane of TiB2, accompanied in cases when the Cu concentration is high by the presence of a further atomic-scale Cu-rich layer. Based on these observations, a possible nucleation sequence for the Al-Cu based alloys with the addition of Al-5Ti-1B grain refiner is proposed whereby Al nucleation occurs on an Ti-rich layer on the surface of TiB2, which can then be preserved from a subsequent peritectic transformation by a surrounding eutectic reaction that forms a Cu-rich layer when sufficient Cu is present. This proposed nucleation sequence should help to link the absence or presence of Al3Ti layers on the basal plane of TiB2 to processing conditions in post-solidification studies. Although it is difficult to know with absolute certainty if a TiB2 particle observed in the post-solidification microstructure was active or inactive for heterogeneous nucleation during the solidification process, this experimental study highlights the possible role of Cu segregation on grain refinement of Al-Cu alloys by the Al-5Ti-1B grain refiner.

Thermally-enhanced microstructures of Si/TiNi film electrodes for improved electrochemical properties

The structural and electrochemical properties of annealed Si film electrodes with titanium–nickel (TiNi) current collectors have been investigated to identify compounds and microstructures that improve electrochemical performances. The as-deposited Si/TiNi electrode consisted of amorphous Si (α–Si) and crystalline B2–TiNi alloy at room temperature. Only a thin oxide layer was formed at the interface, and any significant microstructural change was not found at 500 °C compared to the as-deposited electrode. The Ni atoms started to migrate from the TiNi alloy to the Si film at 600 °C, which resulted in the formation of a Ti-rich (Ti2Ni) layer at the interface. A large amount of Ni diffused into the Si film at 700 °C led to the formation of nickel silicide (NiSi2) particles being dispersed in the film. The Si/TiNi electrodes that were annealed at a temperature exceeding 600 °C showed improved electrochemical properties such as initial efficiency (~90%) and cycle performance (50% capacity retention after 300 cycles). The excellent electrochemical performance was achieved by enhancing the structural stability in annealed Si/TiNi electrodes.

Joining Alumina to Titanium Alloys Using Ag-Cu Sputter-Coated Ti Brazing Filler.

The joining of alumina (Al2O3) to γ-TiAl and Ti6Al4V alloys, using Ag-Cu sputter-coated Ti brazing filler foil, was investigated. Brazing experiments were performed at 980 °C for 30 min in vacuum. The microstructure and chemical composition of the brazed interfaces were analyzed by scanning electron microscopy and by energy dispersive X-ray spectroscopy, respectively. A microstructural characterization of joints revealed that sound multilayered interfaces were produced using this novel brazing filler. Both interfaces are composed mainly of α-Ti, along with Ti2(Ag,Cu) and TiAg intermetallics. In the case of the brazing of γ-TiAl alloys, α2-Ti3Al and γ-TiAl intermetallics are also detected at the interface. Bonding to Al2O3 is promoted by the formation of a quite hard Ti-rich layer, which may reach a hardness up to 1872 HV 0.01 and is possibly composed of a mixture of α-Ti and Ti oxides. Hardness distribution maps indicate that no segregation of either soft or brittle phases occurs at the central regions of the interfaces or near the base Ti alloys. In addition, a smooth hardness transition was established between the interface of Al2O3 to either γ-TiAl or Ti6Al4V alloys.

Microstructural study on Sn–Zn/Cu–Ti diffusion reaction for internal tin Nb3Sn conductor development

To take advantage of Zn and Ti doping effects on improvement of diffusion behavior in the Sn–Cu mixing step and the Nb3Sn layer formation for the internal tin process, a diffusion couple configuration of Nb/Cu–Ti/Sn–Zn has been proposed. In this work, a fundamental study on the diffusion behavior of Sn–Zn/Cu–Ti, with different heat treatments, was conducted to aid in a better understanding of Nb3Sn phase formation, via internal tin diffusion. The first feature is that at 210∘C, the γ-CuZn phase forms at the reaction interface of Sn-20 wt%Zn and Cu, following which the β-CuZn and η-CuSn phases form at 400∘C. Compared with the Sn/Cu-12 wt%Zn system, Kirkendall voids appear to be suppressed, as few ε-CuSn phases, in which many voids grow, form in the Sn–Zn/Cu system. At 550∘C, a dendritic mixed phase of α-CuZn and δ-CuSn phases form. At this temperature, Sn appears to diffuse faster than in the case of the Sn/Cu–Zn system. It is observed that, after heat treatment at 550∘C, very fine compound particles of Sn–Ti are homogeneously distributed in the outer regions originally known to be the Cu–Ti sheath area. In the multifilamentary wires consisting of Nb/Cu–Ti/Sn–Zn diffusion couple configuration, no Ti rich compound layer was formed at the boundary of the Nb sub-elements, which overcomes the problem to suppress a smooth Sn and Ti diffusion into the Nb sub-element, when Ti is doped to Sn cores. Thus, no Ti-rich compound layer segregation, few void formations in the Sn–Cu mixing step and distributed fine Sn–Ti particles in the matrix accounts for the improved Jc characteristics in Nb/Cu–Ti/Sn–Zn wires. With regards to the mechanical properties, alloying of 20 wt% Zn to Sn cores increases the Vickers hardness of the Sn core to approximately 20kgf/mm2, almost double that of pure Sn or Sn-alloys with a small amount of Ti.

Effect of Ti Powders Addition on Mechanical and Metallurgical Properties of IN718/BNi-2/316L Diffusion Couple

Transient liquid bonding (TLP) of IN718 superalloy to 316L alloy was carried out using BNi-2 interlayer with Ti powders. Effect of Ti powders addition with 2 and 4 wt% Ti on metallurgical, diffusion and mechanical properties of joints bonded at 1050 °C for three bonding time 45 min, 2 h and 16 h was investigated. The microstructure of the diffusion zone was investigated by scanning electron microscopy, and the phases in the diffusion zone were analyzed by energy-dispersive spectroscopy. The shear strength and microhardness tests were carried out to investigate the mechanical properties of the joints. The results showed that the presence of eutectic and dendritic solidification increased by Ti addition. In addition to this, a continuous Ti-rich intermetallic layer was formed at the bonding region/316L interface. It was observed that TiB2 component was formed in the TLP joint and it led to reduced Nb boride in bonding region. The formation mechanism and solidification progress path of the IN718/BNi-2 + Ti powder/316L joint were proposed. The hardness in the joint containing Ti powders increased, but it had little effect on shear strength at room temperature.

Morphological changes of the PVD coatings after isothermal annealing

The main of this work is to investigate the impact of the isothermal annealing on the structure and morphological changes on the PVD hard coatings. Three different coatings based on the AlTiN system were deposited onto cemented carbide substrates containing 6 wt% of Co. Two coatings AlTiN/TiAlN and AlTiN/TiN with an initial thickness of 3 μm has a coarse-grained columnar structure with a chemically graded nanomultilayering consisting of alternating Ti-rich and Al-rich layers. In the case of the AlTiN/TiAlN coating, a total of 20 layers were applied. Ten layers had a thickness of 200 nm (AlTiN) and 10 layers with the thickness of 100 nm (TiAlN). The same number of nanolayers was also retained for the AlTiN/TiN multilayer coating. The TiN layers were 100 nm thick. The third coating was a nanocomposite sandwich coating formed by a TiSiN adhesive layer (50nm), a functional nanocomposite AlTiSiN layer (2 μm) and a top functional TiSiN layer with a thickness of 1 μm. The coatings were prepared by cathodic arc evaporation using the LARC® technology. After deposition, the samples were annealed at high temperatures (700 °C, 800 °C, 900 °C, 1000 °C) for one hour in air. With X-ray diffraction and scanning electron microscopy, the morphology, structure and phase composition of coatings and oxide layers after annealing were evaluated.After the heat-treatment at 800 °C, cross-sectional and surface morphology images revealed, that an oxide layer has grown on top of the all coatings. EDS analysis showed that the oxide layers on the multilayers coatings had a layered structure, which was found to be the consequence of the high diffusion rate of Al. The interface with the coatings was very rough. It was depleted of Al, which diffused to the upper surface of the oxide. The oxidation process of nanocomposite AlTiSiN/TiSiN coating started by forming an oxide layer of ternary Ti–Si–O at the interface. This system segregated into two phases of TiO2 and SiO2. The nanocomposite coating remained stable even after annealing at 1000 °C. The surface of the multilayers was accompanied by a network of cracks at this temperature. An interesting phenomenon was also observed on the substrates itself during the annealing. As a result of oxidation, unprotected sides of the substrates expanded. Globular oxides based on tungsten and cobalt were also formed on the surface of the samples at the places where the coating was peeled off. The reasons for the differences are discussed.

Microstructure and properties of ZrO2 ceramic and Ti-6A-4V alloy vacuum brazed by Ti-28Ni filler metal

Reliable ceramics/metal joints have an extensive application in the aerospace and biomedical area. However, ZrO2ceramic has not been investigated systematically compared to the Si3N4and Al2O3ceramic. Therefore, successful brazing of ZrO2ceramic and Ti-6A-4V alloy was achieved by using a binary active Ti-28Ni filler metal in this paper. The effect of holding time on the microstructure of ZrO2 ceramic/filler metal interface and mechanical properties of brazed joints was investigated. The results indicated that the representative interfacial microstructure was ZrO2ceramic/Ti2O/Ni2Ti4O/Ti-rich phase/Ti2Ni+α-Ti. With the increase of holding time, the thickness of Ti-rich layer in the interface of ZrO2/Ti-6Al-4Vjoint decreased obviously due to the diffusion of Ti atoms. Substantial brittle intermetallic compounds Ti2Ni and Ni2Ti4O were formed in the joint, which were detrimental to the mechanical properties of the brazed joints. The maximum shear strength of joint was 112.7 MPa when brazed at 1060 °C for 10 min.

Improved mechanical performance and wear resistance of Ti-coated cBN–WC–Ni composites

Abstract

Effect of Zn addition and Ti doping position on the diffusion reaction of internal tin Nb3Sn conductors

Addition of Zn to a Cu matrix during Cu–Zn/Sn interdiffusion reactions at 400 °C leads to the formation of a solid ternary Cu–Zn–Sn phase, β-CuZn, at the outermost reaction layer next to the porous ε phase. The use of a brass matrix considerably suppresses void formation and promotes homogeneous outward Sn diffusion in pre-annealing, prior to Nb3Sn formation. There are exactly three paths for Ti doping in the internal tin process, i.e. doping to Sn cores, Nb filaments, and Cu matrix. Ti doping to Sn cores causes a Ti-rich layer formation at the boundary of the Nb filament pack; however, no Ti-rich layers are formed as a result of small Ti doping to the matrix and to Nb filaments. The absence of Ti-rich layers is believed to contribute to a smooth Sn diffusion and suppression of void growth. Atom probe tomography measurements reveal that Ti doping to Sn cores leads to a more inhomogeneous Ti distribution near the grain boundary and a larger variation of the grain boundary thickness than doping to Nb filaments, which may contribute to a better Sn grain boundary diffusion. It is concluded that Ti doping to the matrix, instead of doping to Sn cores, might be more effective in maintaining better growth kinetics of Nb3Sn.

The impact of Ni and Mo on growth-morphology and mechanical properties of arc evaporated Ti-Cr-N hard coatings

We studied the growth-morphology, structure, mechanical and tribological properties of Ti0.43Cr0.57N0.92, Ti0.35Cr0.65N0.78/Ni0.003, and Ti0.13Cr0.74Mo0.13N0.71/Ni0.025 coatings synthesized by cathodic arc evaporation. The Ni-free and low-Ni-containing coatings, Ti0.43Cr0.57N0.92 and Ti0.35Cr0.65N0.78/Ni0.003, are characterized by a dense columnar growth-morphology and coherent growth of Ti-rich and Cr-rich layers (due to the two-fold substrate rotation). The addition of only 0.3 at% Ni to the growing film results in a significant grain refinement, increased compressive stresses (from −6.7 to −9.7 GPa), and increased hardness (from 25 to 43 GPa). These two coatings show a high cohesive strength with crack formation during scratch tests for loads above 75 N. The Ni-richer and Mo-containing coating, Ti0.13Cr0.74Mo0.13N0.71/Ni0.025, also exhibits a dense growth-morphology with a layered structure. The higher Ni content of 2.5 at% leads to metallic Ni formation, allowing the growth of films with low compressive stresses of only −0.6 GPa and still a comparably high hardness of 30 GPa. The Ni segregated to grain and column boundaries weakens their cohesive strength (allowing to relax the growth induced stresses), and crack formation can already be observed for loads of 25 N during the scratch tests. However, their performance during pin-on-disk tests at 550 °C is superior – with a lower coefficient of friction of 0.45 and lower wear rate of 0.4 · 10−6 mm3/(N·m) – to the other two coatings.

Self-assembly of correlated (Ti, V)O2 superlattices with tunable lamella periods by kinetically enhanced spinodal decomposition

Spinodal decomposition, the spontaneous phase separation process of periodic lamellae at the nanometer scale, of correlated oxide ((Ti, V)O2) systems offers a sophisticated route to achieve a new class of mesoscale structures in the form of self-assembled superlattices for possible applications using steep metal–insulator transitions. Here, we achieve the tunable self-assembly of (Ti, V)O2 superlattices with steep transitions (ΔTMI < 5 K) by spinodal decomposition with accurate control of the growth parameters without conventional layer-by-layer growth. Abrupt compositional modulation with alternating Ti-rich and V-rich layers spontaneously occurs along the growth direction because in-plane lattice mismatch is smaller in this direction than in other directions. An increase in the film growth rate thickens periodic alternating lamellae; the phase separation can be kinetically enhanced by adatom impingement during two-dimensional growth, demonstrating that the interplay between mass transport and uphill diffusion yields highly periodic (Ti, V)O2 superlattices with tunable lamellar periods. Our results for creating correlated (Ti, V)O2 oxide superlattices provide a new bottom-up strategy to design rutile oxide tunable nanostructures and present opportunities to design new material platforms for electronic and photonic applications with correlated oxide systems.

Microstructure and mechanical properties of laser welded TC4 titanium alloy/304 stainless steel joint with (CoCrFeNi)100-xCux high-entropy alloy interlayer

The feasibility of (CoCrFeNi)100-xCux high-entropy alloy filler metal for laser welding between TC4 titanium alloy and 304 stainless steel is investigated. Reliable metallurgical bonding was achieved between weld zone and 304 stainless steel. The weld zone consisted of ductile FCC solid solution phase and Cu-rich phase which segregated at the grain boundary of FCC phases. The Ti/Cu transition zone is composed of Ti-rich layer near TC4 titanium alloy substrate and Ti-depleted layer adjacent to the weld zone. Ti-rich IMCs and β-Ti phases formed in Ti-rich layer, and Ti(Fe,Co,Cr)2 and Ti(Fe,Ni)2 phases formed in Ti-depleted layer. With increase of Cu content in filler metal, the segregated Cu-rich phase in weld zone increased, and Ti(Fe,Co,Cr)2 and Ti(Fe,Ni)2 phases decreased along with the increase of (Fe,Cr)-rich phase and segregated Cu-rich phase. The tensile strength of the joint with CoCrFeNi filler metal was 158 MPa, and it decreased with increase of Cu in filler metal until the atomic percent of Cu reached 27.27 at. %, but raised to 161 MPa while using (CoCrFeNi)66.67Cu33.33 filler metal. All joints fractured through the transition zone, and the maximum microhardness of the transition zone reached 803 HV, higher than those of substrates and weld zone. The formation of Ti(Fe,Co,Cr)2 and Ti(Fe,Ni)2 phases resulted in the limited tensile strength and high microhardness in the transition zone.

Nanoscale Surface Disorder for Enhanced Solar Absorption and Superior Visible-Light Photocatalytic Property in Ti-Rich BaTiO3 Nanocrystals.

Lattice disorder has emerged as a novel strategy to realize visible-light photocatalytic activity, but many existing studies often involved reduction states simultaneously. Photocatalysts based on only the lattice disorder but without the reduction states are still quite lacking and challenging. To this end, we explored a new type of lattice disorder in terms of the surface atom nonstoichiometry strategy in BaTiO3. Well-dispersed tetragonal BaTiO3 nanocrystals with a uniform size (∼20 nm) and cuboid morphology were hydrothermally synthesized through controlling over t-butylamine and oleic acid. HRTEM coupled with structural evolution analysis reveals the existence of a Ti-rich layer on BaTiO3 nanocrystals with surface atom disorder, which gives an overall Ti/Ba ratio of 1.50:1. This is mainly dominated by the oriented adsorption between oleic acid and surface Ba2+ of the nucleus during solution reaction. Such a surface disorder and Ti-rich nonstoichiometry effect could facilitate the enhanced visible-light absorption with a wavelength span of 400–700 nm that enables the superior visible-light photocatalytic property, which is not subject to the reduction states. This work demonstrates a first white material presenting a new type of lattice disorder that would be helpful for a wide range of photocatalyst explorations.

Titanium carbonitride seeded crystallization of aluminum

The refinement effect of titanium carbonitride on pure aluminum was investigated by adding the mixed powder of Ti and TiCN. Comparing with adding Ti or TiCN separately, 0.3% addition of the mixed refiner of Ti and TiCN has better grain refinement performance and the grain size of pure aluminium can decrease to 120 μm. With the prolongation of holding time, the pure aluminum with TiCN/Ti showed excellent anti-fading performance which can still maintain fine equiaxed grains (180 μm) after holding 8 h. The charictaristics of refined specimens were revealed by OM, FSEM, XRD, and DCS test and the results suggest that TiCN with Ti-rich layer in aluminum melt can reduce undercooling and act as heterogeneous nucleus of α-Al. The refinement mechanism of the composite refiner is the formation of Ti-rich layer on the interface of TiCN which promotes the nucleation of α-Al.

Effect of thermal treatments in high purity Ar on the oxidation and tribological behavior of arc-PVD c-Al0.66Ti0.33N coatings

The effect of temperature in high purity Ar (low oxygen partial pressure) on the oxidation and crystal phase evolution of c-Al0.66Ti0.33N arc-PVD coatings was investigated. High temperature tribology in Ar jet and adhesion behavior of the oxidized coating were addressed. The use of Ar protects the coating from the oxidation reactions and allowed to shed light into the details and paths of the nitride oxidation process. The c-Al0.66Ti0.33N nitrides were slightly oxidized in Ar even at high temperature (700 to 900 °C). The surface chemistry evaluation shows very thin Al- and Ti-based oxide layers formation after treatment at 700 °C in Ar. However, the onset formation of the rutile TiO2 oxide was detected only after treatment at 900 °C and was clearly found by low-angle XRD diffraction after 1000 °C. The XPS surface analysis of the oxidized samples at 800 °C indicated the formation of extremely thin layers mainly composed of mixed oxides (α-Al2O3 γ-Al2O3 and rutile TiO2). The gradual changes in the chemical composition of the oxidized coatings observed at 900 °C clearly demonstrate the formation of gradient boundaries between Al-rich and Ti-rich oxy-nitride layers. After exposure at 1000 °C at least four oxides and oxi-nitrides layers and interlayers were found. Additionally, it is suspected that the transition oxide phases such as c-TiO, c-Al0.54Ti2.46O0.28N4.58, α-Al2TiO5 and TiOxNy might be precursors leading to the formation of thermodynamically stable rutile and alumina phases. Surface inspection after high temperature tribology in Ar jet showed formation of tensile cracks in the wear tracks and wear processes due to high size particle adhesion-abrasion and low size particle micro-ploughing of hard nitrides, oxi-nitrides and brittle oxide particles. Adhesion tests performed after high temperature tribology in Ar showed critical load decrease related to cohesive and adhesive damages at the contact point of the scratch track.

Annealing Induced Interfacial Evolution of Titanium/Gold Metallization on Unintentionally Doped β-Ga2O3

Here, we study the kinetic evolution of the interface between a Ti/Au metal stack and bulk (010) β-Ga2O3 substrate under different annealing conditions using scanning / transmission electron microscopy. We observe distinct processes of interfacial reaction and interdiffusion between the metal films and at the metal-semiconductor junction. Upon rapid thermal annealing (RTA), the as-deposited Ti readily reacts at the β-Ga2O3 interface, driven by redox favorability. After a 1-min 470°C N2 RTA, the interface exhibits two segregated crystalline layers: a ∼5 nm Ti-rich (Ti-TiOx) layer lattice-matched to the β-Ga2O3 substrate and a ∼3 nm Ga-rich (TiGax) layer. A substitutional mechanism is proposed based on the similarity in ionic radii of Ti+3, Ti+4, and Ga+3. After 15-min RTA, the Ga-rich layer is diluted within the Ti-Au matrix, while the Ti-TiOx layer does not significantly change, and there is no further observable Ga out-diffusion from the substrate. Thus, we propose that the Ti-TiOx layer acts as a diffusion barrier, even when it is no longer lattice-matched with β-Ga2O3. In addition, Ti-rich nanocrystals form within the Ti-Au layer, presumably via the proceeding reactions. The observations here provide insights for contact stack evolution during operation of power electronic devices at elevated temperature.