A comparative study of microstructure,oxidation resistance,mechanical, and tribological properties of coatings in Mo–B–(N), Cr–B–(N) and Ti–B–(N) systems

M–B–(N) (M = Mo, Cr, Ti) coatings were obtained by the magnetron sputtering of MoB, CrB₂, TiB, and TiB₂ targets in argon and in gaseous mixtures of argon with nitrogen. The structure and composition of the coatings have been investigated using scanning electron microscopy, glow-discharge optical emission spectroscopy, and X-ray diffraction. The mechanical and tribological properties of the coatings have been determined by nanoindentation, scratch-testing, and ball-on-disk tribological tests. The experiments on estimating the oxidation resistance of coatings were carried out in a temperature range of 600–1000°С. A distinctive feature of TiB₂ coatings was their high hardness (61 GPa). The Cr–B–(N) coatings had high maximum oxidation resistance (900°С (CrB₂) and 1000°С (Cr–B–N)) and possessed high resistance to the diffusion of elements from the metallic substrate up to a temperature of 1000°С. The Mo–B–N coatings were significantly inferior to the Ti–B–(N) and Cr–B–(N) coatings in their mechanical properties and oxidation resistance, as well as had а tendency to oxidize in air atmosphere after long exposure at room temperature. All of the coatings with nitrogen possessed a low coefficient of friction (in a range of 0.3–0.5) and low relative wear ((0.8–1.2) × 10^–6 mm3 N^–1 m^–1.     

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

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-Si₃N₄ 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.     

The Influence of H,W, H/E,H 3/E 2, Structure and Chemical Composition on the Resistance of Ti–B–(N), Mo–B–(N), Cr–B–(N), and Zr–B–(N) Coatings to Cyclic Impact Loading

Protection of Metals and Physical Chemistry of Surfaces - Coatings based on transition metal borides (Ti, Mo, Cr, Zr) were obtained by magnetron sputtering of ceramic targets in Ar and Ar–15%...     

Elevated-temperature tribological behavior of Ti–B–C–N coatings deposited by reactive magnetron sputtering

Quaternary Ti-B-C-N coatings with various carbon contents were deposited on high-speed steel(HSS)substrates by reactive magnetron sputtering(RMS) system.The elevated-temperature tribological behavior of Ti-BC-N coatings was explored using pin-on-disk tribometer,scanning electron microscopy(SEM),and energy-dispersive X-ray spectroscopy(EDX).The present results show that the steady-state friction coefficient value and the instantaneous friction coefficient fluctuation range of TiB-C-N coatings decrease as carbon content increases at100 and 300 ℃,while the steady-state friction coefficient value of all Ti-B-C-N coatings becomes higher than 0.4 at500 ℃.As ambient temperature increases,the running-in periods of all Ti-B-C-N coatings become shorter.Wear damage to Ti-B-C-N coatings during sliding at elevated temperature is mainly caused by adhesive wear,and adhesive-wear damage to Ti-B-C-N coatings increases as ambient temperature increases;however,higher carbon content is beneficial for decreasing the adhesive-wear damage to Ti-B-C-N coatings during sliding at elevated temperature.     

Stability of B2 ordered phase in the Ti-rich portion of Ti–Al–Cr and Ti–Al–Fe ternary systems

The stability region of the B2 phase at 1000°C in the Ti-rich part of the Ti–Al–Cr and Ti–Al–Fe ternary systems are investigated by energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) using two-phase alloys and diffusion couples. It is established that the critical boundaries of the A2/B2 continuous ordering transition are functions of both the Al and Fe or Cr contents, and the phase equilibria between the α₂ and the β and between the β and FeTi (B2) phases are strongly affected by the A2/B2 order–disorder transition. By extrapolating these ternary data to the Ti–Al binary and using the Bragg–Williams–Gorsky approximation a metastable A2/B2 ordering boundary is postulated to exist at 1000°C in the vicinity of 23.5 at%Al in the Ti–Al binary system.     

Modified embedded-atom method interatomic potentials for the Ti–C and Ti–N binary systems

Modified embedded-atom method (MEAM) interatomic potentials for the Ti–C and Ti–N binary systems have been developed using previously developed MEAM potentials of Ti, C and N. The potential parameters were determined by fitting to experimental data on the enthalpy of formation, lattice parameter, elastic constants, thermal linear expansion of NaCl-type TiC and TiN, and dilute heat of solution of carbon and nitrogen atoms in hexagonal close-packed Ti. The potentials can describe fundamental physical properties (structural, elastic, thermal and surface properties) of the alloys well, in good agreement with experimental information or first-principles calculations. The applicability of the potentials to atomistic investigations of interactions between TiC or TiN precipitates and matrix, dislocations or other defects, and their effects on deformation and mechanical behaviors of metallic alloys is discussed.     

Direct synthesis and dehydrogenation properties of NaAlH_4 catalyzed with ball-milled Ti–B

Ball-milled Ti–B-doped sodium aluminum hydride was directly synthesized via mechanical ball-milling of a NaH/Al mixture. The mixture was completely hydrogenated to NaAlH_4 after 70 h under hydrogen pressure of 1 MPa. And higher hydrogen pressure is beneficial for the conversion from NaH/Al mixture to NaAlH_4. The dehydrogenation properties of the as-synthesized Ti–Bdoped sodium aluminum were systematically investigated.The result shows that ball-milled Ti–B has a remarkable catalytic effect on the enhanced dehydrogenation properties of NaAlH_4. As-synthesized Ti–B-doped NaAlH_4 sample releases hydrogen at the temperature of about 100 ℃. Approximately 4.15 wt% H_2 is released from ballmilled Ti–B-doped NaAlH_4 at 233.7 ℃. Even at 110 ℃, it also releases about 2.83 wt% hydrogen. The apparent activation energy(E_a) for the first step is estimated to be 83.97 k J·mol~(-1) using Arrhenius equation. Thus, utilization of ball-milled Ti–B as catalyst would substantially enhance the practical applications of NaAlH_4 for hydrogen storage.     

Microstructure and mechanical performance of brazed joints of Cf/SiC composite and Ti alloy using Ag–Cu–Ti–W

Abstract

C_f/SiC composite was brazed to Ti alloy using interlayer of Ag–Cu–Ti–W mixed powder. The effects of W content and brazing parameters on the microstructure and properties of the brazed joints were investigated. The results show that W grains mainly distribute in Ag phase in the brazing layer and provide the effects of reinforcement and lowering residual thermal stress on the joint. The room temperature and 500°C shear strengths of the joints performed at 500°C for 30 min with Ag–Cu–Ti–50W (vol.-%) are remarkably higher than the optimal strengths of the joints brazed with Ag–Cu–Ti.     

Structure and properties of selected (Cr–Al–N,TiC–C,Cr–B–N) nanostructured tribological coatings

The paper will present the state-of-art in the process, structure and properties of nanostructured multifunctional tribological coatings used in different industrial applications that require high hardness, toughness, wear resistance and thermal stability. The optimization of these coating systems by means of tailoring the structure (graded, superlattice and nanocomposite systems), composition optimization, and energetic ion bombardment from substrate bias voltage control to provide improved mechanical and tribological properties will be assessed for a range of coating systems, including nanocrystalline graded Cr_1−xAl_xN coatings, superlattice CrN/AlN coatings and nanocomposite Cr–B–N and TiC/a-C coatings. The results showed that the superlattice CrN/AlN coating exhibited a super hardness of 45 GPa when the bilayer period Λ was about 3.0 nm. Improved toughness and wear resistance have been achieved in the CrN/AlN multilayer and graded CrAlN coatings as compared to the homogeneous CrAlN coating. For the TiC/a-C coatings, increasing the substrate bias increased the hardness of TiC/a-C coatings up to 34 GPa (at −150 V) but also led to a decrease in the coating toughness and wear resistance. The TiC/a-C coating deposited at a −50 V bias voltage exhibited an optimized high hardness of 28 GPa, a low coefficient of friction of 0.19 and a wear rate of 2.37 × 10⁻⁷ mm³ N⁻¹ m⁻¹. The Cr–B–N coating system consists of nanocrystalline CrB₂ embedded in an amorphous BN phase when the N content is low. With an increase in the N content, a decrease in the CrB₂ phase and an increase in the amorphous BN phase were identified. The resulting structure changes led to both decreases in the hardness and wear resistance of Cr–B–N coatings.     

TEM study of structural and microstructural characteristics of a precipitate phase in Ni-rich Ni–Ti–Hf and Ni–Ti–Zr shape memory alloys

The precipitates formed after suitable thermal treatments in seven Ni-rich Ni–Ti–Hf and Ni–Ti–Zr high-temperature shape memory alloys have been investigated by conventional and high-resolution transmission electron microscopy. In both ternary systems, the precipitate coarsening kinetics become faster as the Ni and ternary element contents (Hf or Zr) of the bulk alloy are increased, in agreement with the precipitate composition measured by energy-dispersive X-ray microanalysis. The precipitate structure has been found to be the same in both Hf- and Zr-containing ternary alloys, and determined to be a superstructure of the B2 austenite phase, which arises from a recombination of the Hf/Zr and Ti atoms in their sublattice. Two different structural models for the precipitate phase were optimized using density functional theory methods. These calculations indicate that the energetics of the structure are not very sensitive to the atomic configuration of the Ti–Hf/Zr planes, thus significant configurational disorder due to entropic effects can be envisaged at high temperatures. The precipitates are fully coherent with the austenite B2 matrix; however, upon martensitic transformation, they lose some coherency with the B19′ matrix as a result of the transformation shear process in the surrounding matrix. The strain accommodation around the particles is much easier in the Ni–Ti–Zr-containing alloys than in the Ni–Ti–Hf system, which correlates well with the lower transformation strain and stiffness predicted for the Ni–Ti–Zr alloys. The B19′ martensite twinning modes observed in the studied Ni-rich ternary alloys are not changed by the new precipitated phase, being equivalent to those previously reported in Ni-poor ternary alloys.     

Thermodynamic model for precipitation of carbonitrides in microalloyed steels and its application in Ti–V–C–N system

Based on mass balance and solubility product equations, a thermodynamic model enabling the calculation of equilibrium carbonitride composition and relative amounts as a function of steel composition and temperature was developed, which provides a method to estimate the carbonitride complete dissolution temperature for different steel compositions. Actual carbonitride precipitation behavior was further verified in Ti–V–C–N microalloyed steel system. The model suggests that for higher [V] and [Ti] dissolved in steels, it is available to decrease the addition of C and N during alloy composition design. The resultant longer fatigue life of the modified steel could be attributed to the more [V] and [Ti] dissolved in the matrix, inducing finer dispersion of carbonitrides. Therefore, this model is proved to be effective in determining better chemical composition for high-performance steels, leading to possible reductions in the cost of production and improvements in the combined mechanical properties of the steels.     

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys have been studied. In order to optimize ductility and strength of the orthorhombic alloys with the nominal compositions of Ti–22Al–23Nb–3Ta and Ti–22Al–20Nb–7Ta, various thermomechanical processing steps were implemented as part of the processing route. With a special heat treatment before rolling to obtain a fine and homogeneous rolled microstructure, the rolled microstructure resulted in a good combination of high tensile yield strength and good ductility of the alloys through available solution and age treatments. The duplex microstructure with equiaxed α₂/O particles and fine O phase laths in a B2 matrix, deforming in α₂+B2+O phase field and treating in O+B2 phase field, possesses the highest tensile properties. The R.T. yield strength and ductility of the Ti–22Al–20Nb–7Ta alloy are 1200 MPa, and 9.8% respectively. The yield strength and ductility values of 970 MPa and 14% were also maintained at elevated temperature (650°C).     

Enhanced Oxidation Resistance of Mo–Si–B–Ti Alloys by Pack Cementation

As new high-temperature structural materials, Mo–Si–B alloys satisfy several requirements such as oxidation and creep resistance. Recently, novel Ti-rich Mo–Si–B alloys have shown an increased creep resistance compared to Ti-free alloys. However, due to the formation of a duplex SiO₂–TiO₂ oxide layer, which allows for fast ingress of oxygen, the oxidation resistance is poor. To improve the oxidation resistance, a borosilicate-based coating was applied to a Mo–12.5Si–8.5B–27.5Ti (in at.%) alloy. After co-deposition of Si and B by pack cementation at 1000 °C in Ar, a conditioning anneal at 1400 °C is used to develop an outer borosilicate layer followed by an inner MoSi₂ and Mo₅Si₃ layer. During both isothermal and cyclic oxidation after an initial mass loss during the first hours of exposure, a steady state is reached for times up to 1000 h at temperatures ranging from 800 to 1200 °C, demonstrating a significantly enhanced oxidation resistance.     

Microstructures and hydrogen embrittlement of Ti–49Al alloy

Microstructures and hydrogen embrittlement of Ti–49 at% Al were investigated. Results showed that there were three kinds of microstructures formed by heat treatment at 1423, 1573 and 1703 K. The specimens which were heat-treated at 1573 K, showed better elongation than the others on tensile tests at room temperature, and this kind of specimens were used to investigate the effect of hydrogen on tensile properties of this alloy. After heat-treatment in hydrogen gas at 1073 K for 3 h, the specimens were divided into three groups. In the first group, they were tensile-tested at room temperature in vacuum; in the second group, they were heat-treated at 823 K for 1.5 h in argon gas followed by tensile-testing at room temperature in vaccum; and in the third group, they were tensile-tested at 473 or 573 K in vacuum. Results showed that for the specimens precharged with hydrogen, the elongation was decreased significantly at room temperature, and that the decreased elongation was recovered by removing hydrogen at 823 K in Ar gas, although it was not recovered to that of the specimens without hydrogen. This means that hydrogen decreases the room temperature elongation of this alloy. For the precharged specimens the elongation was also decreased at 473 and 573 K in vacuum. This may indicate that hydrides in the precharged specimens affect the tensile properties in vacuum.     

Heat Resistance,High-Temperature Tribological Characteristics,and Electrochemical Behavior of Arc-PVD Nanostructural Multilayer Ti–Al–Si–N Coatings

The present paper has aimed at studying heat resistance, electrochemical behavior, and tribological characteristics at high temperatures of superhard (~48 ± 2 GPa), multilayered with a modulation period of 17–18 nm, and nanostructured (nc)AlN-(am)Si₃N₄/(nc)TiN coatings obtained with an ion-plasma vacuum arc. The heat resistance of the coatings studied in the temperature range of up to 800°C inclusive was mainly determined by the oxidation of their surface layers without the substrate intrusion. Having a high coefficient of friction from 0.6 at 20°C to 0.8–0.85 at elevated temperatures, the coatings are characterized by virtually no wear, which was confirmed by profilometry measurements of friction zones. The obtained results concerning electrochemical behavior indicate that the Ti–Al–Si–N coatings are highly efficient in the protection of a cutting tool from corrosion in both acidic and alkaline media.     

Thermal Electrochemical, and Plasma Oxidation of Ti–50Zr, Cu–50Zr, Cu–50Ti, and Cu–33Ti–33Zr Alloys

The three main methods for oxidation of metallic substrates, thermal, anodic and plasma have been applied to a copper, titanium, and zirconium alloy and its corresponding binaries (Cu–33Ti–33Zr, Cu–50Ti, Cu–50Zr and Ti–50Zr). Polished polycrystalline samples of these alloys were examined before treatment, after vacuum thermal annealing at 100°C and heating in 20 mTorr oxygen at 100, 200, and 300°C. ISS depth profiles were taken of selected samples. The least-noble component oxidizes first, but at high temperatures and with plasma oxidation the noble component segregates to the surface. A comparison of the resulting structures on the ternary and binary alloys with different oxidation methods is used to explore the physico-chemical processes during oxidation. Results from these three methods are discussed in terms of physical/chemical parameters that influence the chemical nature and structure of the resulting oxides. The electrochemical processes that occur during the materials reaction with a chosen environment are used to discuss the physical and chemical mechanisms involved. Intrinsic (thermal and plasma oxidation) and extrinsic (electrochemical oxidation) electric fields are shown to influence the chemical and structural nature of the resulting oxide structures. The influence of transport phenomena is discussed.     

Microstructures of Ti–Ni–Fe wire after severe cold-drawing and annealing

The microstructures and mechanical properties of Ti–47 at%Ni–3 at%Fe shape memory alloy wire under the condition of severe cold-drawing at room temperature and different postdeformation annealing processes were intensively investigated using transmission electron microscope(TEM),X-ray diffraction(XRD),Vickers microhardness tester and electron tensile tester.It is indicated that the structure of the alloy evolves into a predominant amorphous structure with a trace of nanocrystalline B2 phase after the cold-drawing of 76%areal reduction.Postdeformation annealing process exerted significant influence on the microstructure and mechanical properties.Crystallization occurs when the cold-drawn wire was annealed at 300℃ for 30 min.The ultimate tensile strength and ductility as well as the superelasticity of the wire are improved significantly by cold-drawing plus postdeformation annealing.     

Properties and fracture processes of Si3N4/ Si3N4 joints brazed using Cu–Zn–Ti filler alloy

Abstract

Si₃N₄ ceramic was jointed to itself using a filler alloy of Cu-Zn-Ti at 1123-1323 K for 0.3-2.7 ks. Ti content in the Cu-Zn-Ti filler alloy was varied from 5 to 20 at.-%. The effect of brazing parameters, such as brazing temperature, holding time and Ti content, on the mechanical properties and facture processes of the Si₃N₄/Si₃N₄ joint were investigated. The results indicated that the increased brazing temperature, holding time and Ti content increase the thickness of the interfacial reaction zone in the Si₃N₄/filler alloy, and the size and amount of the reaction phases in the filler alloy. Their increases lead to increasing shear strength of the joint. The fracture behaviour of the Si₃N₄/Si₃N₄ joint greatly depends on the microstructure of the joint. A suitable thick reaction zone with reaction phases yields the high strength of the Si₃N₄/ Si₃N₄ joint.     

Processing and properties of C/Si–B–C–N fiber–reinforced ceramic matrix composites prepared by precursor impregnation and pyrolysis

The fabrication of C/Si–B–C–N fiber–reinforced ceramic matrix composites (FRCs) is reported. The processing basically involves three individual steps: (i) vacuum-assisted infiltration of a liquid two-component Si–B–C–N precursor into stacked woven carbon fiber fabrics, (ii) stepwise thermal cross-linking and (iii) thermal transformation of the polymer matrix composite into a ceramic matrix composite. Repeated polymer infiltration/pyrolysis cycles resulted in FRCs with 89% relative density. The average bending strength of the FRCs was 255 MPa and considerable fiber pull-out was observed throughout the fractured surfaces. The composite retained its strengths upon bending even at 1500 °C in argon and there was no evidence for brittle fracture. The flexural creep strain of the composite at 1400 °C was 0.55%.