Microstructure,mechanical properties,and high-temperature oxidation resistance of AlN/SiO<Subscript>2</Subscript> nanomultilayer coatings

The microstructure, mechanical properties, and high-temperature oxidation resistance of AlN/SiO₂ nanomultilayer coatings with various SiO₂ layer thicknesses were investigated using X-ray diffractometry, high-resolution transmission electron microscopy, scanning electron microscopy, and nanoindentation. The results revealed that SiO₂ formed wurtzite-typed hexagonal pseudo-crystal structures and grew epitaxially with AlN when its thickness was less than 0.6 nm. Meanwhile, the multilayer coatings yielded superhardness effect with a maximum hardness of 29.0 GPa. A minute increase in SiO₂ thickness from 0.6 to 0.9 nm would decrease the hardness of the nanomultilayer coatings due to the formation of amorphous SiO₂ and destruction of epitaxial structure. The high hardness of superhard coatings was sustained after high-temperature annealing treatment of up to 800°C. However, a further increase in annealing temperature to 900°C caused severe oxidation of AlN and thus degradation of coating’s hardness.     

Structure,mechanical properties and thermal stability of Ti1-xSixN coatings

Ti_1-xSi_xN coating is a promising candidate for wear resistant applications due to their super-hardness and high thermal stability. Here, we explored the structure, mechanical properties and thermal stability of Ti_1-xSi_xN (x?=?0, 0.13, 0.17 and 0.22) coatings deposited by cathodic arc evaporation. Monolithically grown Si-containing Ti_1-xSi_xN coatings, which are Si-solution in TiN for x?=?0.13 and 0.17, reveal a high hardness of 39.4?±?0.67 and 40.6?±?0.72?GPa, respectively. Then Ti_1-xSi_xN transforms into a nanocomposite structure consisting of cubic Ti(Si)N nanocrystallite enveloped by the amorphous SiN_x tissue phase for x?=?0.22, which exhibits a high hardness of 40.0?±?0.6?GPa. However, increasing of Si content leads to a significant increase in compressive stress from ?0.63?GPa for x?=?0 to ?3.78?GPa for x?=?0.13 to ?4.54?GPa for x?=?0.17 to ?5.51?GPa for x?=?0.22. The hardness of Ti_1-xSi_xN coatings can be maintained up to ~ 1000?°C due to the suppressed grain growth, and then decreases for further elevated annealing temperature, whereas the TiN coating exhibits a continuous drop in hardness towards its intrinsic value of ~ 21.3?GPa.     

Effect of extended annealing cycles on the thermal conductivity of AlN/Y2O3 ceramics

The effect of extended annealing cycles (up to 50 h at 1800°C) on the thermal conductivity of polycrystalline AlN, doped with 5 wt% Y₂O₃, has been studied. The microstructural evolution upon annealing has also been characterized in detail, using quantitative scanning electron microscopy (SEM) observation and energy dispersive X-ray analysis (EDX). As-sintered AlN/Y₂O₃ composites typically contained a dilute yttrium aluminate secondary phase well distributed and completely wetting the AlN grains. Upon annealing, the AlN matrix grains isotropically grew, while the grain-boundary yttrium aluminate phase tended to segregate to triple grain junctions. This segregation process produced a collapse of the grain-boundary film thickness, thus resulting in a completely different AlN microstructure dispersed with isolated yttrium aluminate grains. Equilibrium of the microstructural morphology was achieved after annealing times in the interval 5–10 h. As a consequence of microstructural changes, the thermal conductivity of the annealed AlN polycrystal exceeded that of the as-sintered material. A discussion is given about the variation of thermal properties in terms of both segregation to the triple-grain junctions of the intergranular Y₂O₃-phase and grain-growth of the bulk AlN grains.     

Thermal stability and oxidation resistance of V-alloyed TiAlN coatings

V-containing nitride coatings recently attract a wide range of research interests owing to their excellent tribological properties. To evaluate their comprehensive properties, a comparative study on the intrinsic thermal stability and oxidation resistance of TiAlN and TiAlVN coatings are conducted here. Ti_0.56Al_0.44N, Ti_0.50Al_0.44V_0.06N, and Ti_0.40Al_0.50V_0.10N coatings, deposited by cathodic arc evaporation, exhibit a single-phase face-centered cubic structure with a hardness of 28.9–29.8 GPa. The V-containing coatings show a pronounced age-hardening upon annealing, which contributes to a hardness increase of 3.7 and 4.8 GPa at 800 °C for Ti_0.50Al_0.44V_0.06N and Ti_0.40Al_0.50V_0.10N, respectively, corresponding to 2.9 GPa for Ti_0.56Al_0.44N. Also, alloying with V retards the formation of wurtzite AlN upon annealing, especially in Ti_0.50Al_0.44V_0.06N, and thus contributes to a higher hardness above 30 GPa even annealing at 1100 °C, while the hardness of Ti_0.56Al_0.44N significantly reduces to 27.8 ± 0.6 GPa. However, alloying with V into TiAlN leads to an earlier formation of rutile TiO₂ and also Ti-rich oxide top-layer on the outside surface instead of dense Al₂O₃, and thus degrades the oxidation resistance. When exposed to air at 700 °C for 10 h, the Ti_0.50Al_0.44V_0.06N and Ti_0.40Al_0.50V_0.10N coatings suffer from a severe oxidation, whereas only a compact oxide scale with a thickness of ~ 80 nm for Ti_0.56Al_0.44N is formed.     

Highly transparent LiAlON ceramic prepared by reaction sintering and post hot isostatic pressing

A highly transparent polycrystalline LiAlON ceramic with the size of Φ57?mm?×?6?mm was successfully fabricated by reaction sintering (1750?°C, 20?h) and post hot isostatic pressing (HIP, 1850?°C, 3?h, 180?MPa) using AlN, Al₂O₃ and LiAl₅O₈ powders. Related mechanism on the reaction sintering and densification were studied via the analysis of phase and microstructural evolution. High transparency was resulted from full elimination of Al₂O₃ secondary phase and residual pores. It has excellent optical transparency from the visible to middle infrared (IR) bands with the maximum transmittance of ～ 85.5%. The flexural strength and Vikers hardness reach ～303?MPa and ～15.0?GPa, respectively.     

Mechanical Properties and Microstructural Characterization of Amorphous SiCxNy Thin Films After Annealing Beyond 1100°C

The effect of thermal annealing on structure and mechanical properties of amorphous SiC_xN_y (y ≥ 0) thin films was investigated up to 1500°C in air and Ar. The SiC_xN_y films (2.2–3.4 μm) were deposited by reactive DC magnetron sputtering on Si, Al₂O₃ and α‐SiC substrates without intentional heating and at 600°C. The SiC target with small excess of carbon was sputtered at various N₂/Ar gas flow ratios (0–0.48). The nitrogen content in the films changes in the range 0–43 at.%. Hardness and elastic modulus (nanoindentation), change in film thickness, film composition, and structure (Raman spectroscopy, XRD) were investigated in dependence on annealing temperature and nitrogen content. All SiC_xN_y films preserve their amorphous structure up to 1500°C. The hardness of all as‐deposited and both air‐ and Ar‐annealed SiC_xN_y films decreases with growth of nitrogen content. The annealing in Ar at temperatures of 1100°C–1300°C results in noticeable hardness growth despite the ordering of graphite‐like structure in carbon clusters in nitrogen free films. Unlike the SiC, this graphitization leads to hardness saturation of SiCN films starting above 900°C, especially for films with higher nitrogen content (deposited at higher N₂/Ar). This indicates the practical hardness limit achievable by thermal treatment for SiC_xN_y films deposited on unheated substrates. The ordering in carbon phase is facilitated by the presence of nitrogen in the films and its extent is controlled by the N/C atomic ratio. The suppression of graphitization was observed for N/C ranging between 0.5–0.7. Films deposited at 600°C show higher hardness and oxidation resistance after annealing in comparison with those deposited on unheated substrates. Hardness reaches 40 GPa for SiC and ~28 GPa for SiC_xN_y (35 at.% of nitrogen). Such a high hardness of SiC film stems from its partial crystallization. Annealing of SiC_xN_y film (35 at.% of N) in Ar at 1400°C is accompanied by formation of numerous hillocks (indicating heterogeneous structure of amorphous films) and redistribution of film material.     

Structural transformations of boron trioxide under high pressure and high temperature

A systematic study of boron trioxide under high pressure and high temperature (HPHT) was conducted using a Chinese multi-anvil high-pressure apparatus (CHPA). The HPHT phase diagram was determined using X-ray diffraction measurements. Under high pressure (3.6–5.5?GPa) and low temperature (below 450?°C), the boron trioxide grains were reduced to the nanometer size and the hardness reaches to 13.9?GPa (5.5?GPa and 450?°C). The boroxol rings were produced only in the glass phase that was transformed from the α-B₂O₃ phase under HPHT. And the formation mechanism of boroxol rings was discussed according to Raman spectrum and crystal structure of α-B₂O₃ and β-B₂O₃.     

High pressure sintering of cubic boron nitride compacts with Al and AlN

Cubic boron nitride (cBN) compacts, using 15 wt.% Al and 20 wt.% AlN respectively as additives, were sintered in the temperature range of 1300–1700 °C for 20 min under high pressure of 5.0 GPa. The hardness, microstructure, phase composition and cutting performance of the high pressure sintered samples were investigated. A liquid phase sintering and reaction process was observed in the cBN–Al system, which leads to the formation of AlN and AlB₂ as confirmed by X-ray diffraction (XRD) in the sintered compacts. Scanning electron microscopy (SEM) analysis shows that the samples have a homogeneous microstructure. The hardness decreases with increase of sintering temperature and reaches the highest Vickers hardness of 32.1 GPa at 1350 °C. While in the cBN–AlN system, AlN grains agglomerate heavily at temperature below ~ 1500 °C. As the sintering temperature increasing, Al₂O₃ appeared and the AlN agglomeration disappeared gradually. A highest cBN–AlN composite hardness of 29 GPa was achieved when sintered at 1600 °C. Turning tests showed that cBN compacts with 15 wt.% Al as the additive has a longer tool life as compared to that with 20 wt.% AlN. Our results indicated that cBN–Al system is more favourable to obtain well-sintered cBN compacts comparing with the cBN–AlN system.     

Wide Color Variation in SiNO Thin Films Dispersed with Precipitated TiN Nano Particles

Amorphous thin films of Ti_1?ySi_y(N,O) with y ≥ 0.38 were prepared by reactive sputter deposition in a nitrogen atmosphere. Thermal annealing of the films in an ammonia flow above 800°C yielded Si(N,O) amorphous thin films dispersed with precipitated TiN nanosized particles. The film color changed with Si content y and the annealing conditions, from carrot orange to cream yellow in the as‐deposited films due to their oxynitride nature, and from dark green to canary yellow and from iron blue to horizon blue at respective annealing temperatures of 800°C and 900°C due to metallic nature of the TiN nanosized particles precipitated in the annealing.     

Spark plasma sintering of LaB6-(Ti,Zr)B2 composites

The LaB₆-(Ti_, Zr)B₂ composite was fabricated from LaB₆, TiB₂ and ZrB₂ powders by spark plasma sintering (SPS) at 1600–1900°C holding for 5?min under 40?MPa. The densification behaviour, microstructure, mechanical properties were investigated. The complete solid solution phase of (Ti, Zr)B₂ was identified. The morphologies of LaB₆ and (Ti, Zr)B₂ grains were equiaxed and elongated, respectively. The highest relative density of 98.43% and Vickers hardness of 19.56?GPa were obtained at 1900°C. The fracture toughness of 4.43?MPa?m^1/2 was obtained at 1800°C.     

Nano- versus macro-hardness of liquid phase sintered SiC

Silicon carbide polycrystalline materials were prepared by liquid phase sintering. Different rare-earth oxides (Y₂O₃, Yb₂O₃, Sm₂O₃) and AlN were used as sintering additives. The final microstructure consists of core–rim structure owing to the incorporation of AlN into the rim of SiC grains by solid solution. Nano- versus macro-hardness of polycrystalline SiC materials were investigated in more details. The nano-hardness of SiC grains was in the range of 32–34 GPa and it depends on the chemical compositions of grains. The harness followed the core–rim chemistry of grains, showing lower values for the rim consisting of SiC–AlN solid solution. The comparison of nano- and macro-hardness showed that nano-hardness is significantly higher, generally by 5–7 GPa. The macro-hardness of tested samples had a larger scatter due to the influence of several factors: hardness of grains (nano-hardness), indentation size effect (ISE), microstructure, porosity, and grain boundary phase. The influence of grain boundary phase on macro-hardness is also discussed.     

Self-lubricating behavior caused by tribo-oxidation of Ti3SiC2/Cu composites in a wide temperature range

Ti₃SiC₂/Cu composite, a new wide temperature range intelligent lubricating functional material, was fulfilled, for mechanical equipment components, by Spark Plasma Sintering process. The microstructure, composition and mechanical properties of the Ti₃SiC₂/Cu composites (TSC-Cu) were investigated. Additionally, the friction and wear behaviors of TSC-Cu sliding against Inconel 718 were conducted on a pin-on-disk configuration at a sliding speed of 0.5 m/s under a load of 5 N at 25–800 °C. For comparison, the tribological property of polycrystalline Ti₃SiC₂/Inconel 718 was measured in an identical condition. The worn surface of TSC-Cu was analyzed by SEM, EDS and XPS, respectively. The results indicated that TSC-Cu consisted of Ti₃SiC₂, TiC and Cu₃Si. It was worth noting that the as-formed Cu₃Si uniformly distributed along the grain boundary of Ti₃SiC₂. As for mechanical property, the addition of Cu increased the hardness, compressive strength of TSC-Cu but lowered its flexural strength. Compared with polycrystalline Ti₃SiC₂, the average friction coefficient of TSC-Cu was higher at 25–400 °C whereas it was lower at 600 °C and 800 °C. The lower friction coefficient was owing to the cooperative lubricating characteristic of tribo-oxidation films containing TiO₂, SiO₂ and CuO. Furthermore, the wear rate of TSC-Cu was absolutely lower than that of polycrystalline Ti₃SiC₂, which resulted from the effective surface strengthening effect of the as-formed hard TiC product. Moreover, the wear mechanism of the composite changed from three-body abrasion wear to adhesion wear and tribo-oxidation wear, with the temperature increasing from RT to 800 °C.     

Effect of controllable decomposition of MAX phase (Ti3SiC2) on mechanical properties of rapidly sintered polycrystalline diamond by HPHT

Sintered polycrystalline diamond (PCD) was prepared via high temperature and high pressure (HTHP) process using Ti₃SiC₂ and Si as the binder. The effect of decomposition of Ti₃SiC₂ on comprehensive properties of PCD was investigated at different temperatures. Results show that Si formed liquid phase infiltrated diamond surface at high temperatures to inhibit diamond graphitization and to reduce defects significantly. In addition, strong covalent bond of Si-diamond was generated, which increased the strength of PCD. At suitable temperature, Ti₃SiC₂ would partially decompose to TiC and SiC with high activity. Strong covalent bond of TiC and SiC also increased relative density and hardness of PCD, and residual Ti₃SiC₂ enhanced the toughness of PCD. At 1500 °C, the hardness and toughness of PCD reached 54.35 GPa and 8.6 MPa m^1/2, respectively, which are 19% and 82%, respectively, higher than PCD sintered at 1350 °C.     

Influence of annealing temperature on microstructure evolution of TiAlSiN coating and its tribological behavior against Ti6Al4V alloys

TiAlSiN multicomponent coating, owing to its high hardness and excellent high temperature resistance, was widely used in the cutting field of difficult-to-cut materials such as titanium alloys. For machining titanium alloys, high temperature is easy to gather on the tool chips and deteriorate the cutting tools. Moreover, high temperature will also promote the microstructure evolution and make the wear mechanism more complex. In this paper, TiAlSiN coatings were deposited on cemented carbides and annealed at 400 °C, 600 °C and 800 °C respectively for 60 min in air, followed by reciprocating friction tests against Ti6Al4V counterparts. AFM, SEM, EDS and XPS were applied to investigate the microstructure evolution and tribological behavior of TiAlSiN coating after high temperature annealing. The results demonstrated that the oxidation resistance of TiN phase in TiAlSiN coating was worse than Si₃N₄ and AlN phases. These nitrides can be oxidized to TiO₂, SiO_x and AlO_x under 600 °C, and the depth of oxide layer was increased with the rising annealing temperature, resulting in the coarsened microstructure. The wear mechanisms of as-deposited TiAlSiN coating were oxidation wear and adhesion wear. With the rising annealing temperature, abrasive wear was gradually enhanced. For the TiAlSiN coating annealed at 800 °C, abrasive wear became the dominant wear mechanism.     

High-pressure work hardening of alumina

We report a method for strengthening microcrystalline alumina compaction by work hardening under conditions of high pressure and temperature, in which the hardness significantly improved. Micro-sized spherical alumina powders were treated without additives under 5.5 GPa–14 GPa and 600 °C–1200 °C. A polycrystalline sample sintered at 12 GPa and 900 °C yielded a Vickers hardness of 30.0 GPa, which was 1.5 times that of single-crystal alumina and the highest value reported to date for alumina. By analysing the density, hardness, and microstructure of the sintered samples, the results demonstrated that under ultra-high pressure and below the recrystallisation temperature of alumina, plastic deformation of the grains produced many substructures such as stacking faults and lattice distortions showing obvious work hardening effects. Combined with well-bonded crystalline and amorphous grain boundaries, the hardness of the samples was unexpectedly enhanced.     

Microstructure,mechanical and electrochemical properties of Ti3AlC2 coatings prepared by filtered cathode vacuum arc technology

Ti₃AlC₂ MAX phases have attracted increasing attention due to their unique properties. However, high synthesis temperatures of Ti₃AlC₂ bulk materials limit their further development. In this work, Ti₃AlC₂ coatings were prepared by a two-step method with filtered cathode vacuum arc (FCVA) deposition at room temperature and annealing at 800 °C for 1 h. The structure and properties of coatings were investigated. The results showed that the formation of Ti₃AlC₂ phase in the annealed coating depended on the C₂H₂ flow rate during deposition. At low C₂H₂ flow rates (≤ 9 sccm), almost no Ti₃AlC₂ phase was formed. As the C₂H₂ flow rate increased, the annealed coatings mainly exhibited Ti₃AlC₂ phases, the texture of which transformed from (104) to (105) planes. Meantime, the hardness of Ti₃AlC₂ coatings continuously increased to a maximum of 20.7 GPa, and the corrosion resistance first increased and then decreased with the increase of C₂H₂ flow rate.     

Reversible phase transformation in Ti2AlC films during He radiation and subsequent annealing

Ti₂AlC film can be used as a protective coating for fuel cladding materials and structural materials in nuclear reactors. However, the related radiation damage and the helium (He) effects have not been well understood. In this work, the He radiation effects on Ti₂AlC thin films, deposited by reactive magnetron sputtering, were studied. In addition to the detailed characterization of the radiation-induced defects and He bubbles, phase transformation was identified and investigated during film deposition, ion irradiation, and subsequent annealing. Results suggested that the hexagonal close-packed (hcp) Ti₂AlC was formed from a solid-solution face-centered cubic (fcc) (Ti₂Al)C phase during the film deposition process. A phase transformation from hcp-Ti₂AlC to fcc-(Ti₂Al)C happened during the He ion irradiation, while a reversible phase transformation from fcc-(Ti₂Al)C to hcp-Ti₂AlC occurred during the post annealings at temperatures above 600 °C. The reversible phase transformation indicates dynamic restoration of this material and provides insights into the design of new irradiation-damage-tolerant ceramic materials.     

Microstructure evolution and plastic mechanism in deformation of bulk amorphous Al2O3-ZrO2-Y2O3

In this work, compressive deformation is performed on bulk amorphous Al₂O₃-ZrO₂-Y₂O₃ at moderate temperatures. The amorphous samples display brittle fracture without any noticeable permanent strain at 500 °C. However, a large plastic strain of up to 15.1% is achieved at 600 °C. During the entire compressive deformation process, the samples remain amorphous, and shear bands start to form, accompanied by a stress drop. The amorphous AZY shows low Vickers hardness value of 2.8 GPa at 500 °C, and 2.2 GPa at 600 °C, due to the disordering microstructure. In the optical microscope images, local plastic deformation are detected around the indention without large cracks. Transmission electron microscopic observations and selected area electron diffraction analysis suggest that the shear band formation originates from the presence of free volume. Furthermore, the nucleation and propogation of shear bands lead to the large macroscopic plastic strain in the bulk amorphous Al₂O₃-ZrO₂-Y₂O₃.     

HiPIMS induced high-purity Ti3AlC2 MAX phase coating at low-temperature of 700 °C

Ti₃AlC₂, one of Ti-Al-C MAX phases, has received extensive attention due to its unique nano-laminated structure and combined properties of metals and ceramics. However, ultra-high synthesis temperature exceeding 800 °C is a critical challenge for broad application of Ti₃AlC₂ coatings on temperature-sensitive substrates. In this study, Ti-Al-C coatings were deposited on Ti-6Al-4V substrates using high-power impulse magnetron sputtering (HiPIMS) and DC sputtering (DCMS) for comparison. Different from as-deposited amorphous Ti-Al-C coating by DCMS, nanocrystalline TiAl_x compound was achieved by HiPIMS deposition due to highly ionized plasma flux with high kinetic energy. Furthermore, HiPIMS promoted the generation of dense and smooth Ti₃AlC₂ phase coating after low-temperature annealing at 700 °C, while annealed DCMS coating only obtained Ti₂AlC. In-situ XRD demonstrated such Ti₃AlC₂ phase could be early involved in crystallization at 450 °C, lowest than synthesis temperature ever reported. The mechanical properties of Ti₃AlC₂ coating were also discussed in terms of structural evolution.     

Influence of heat treatment on alternant-layer structure and mechanical properties of Al2O3-TiO2-MgO coatings

Al₂O₃-TiO₂-MgO ceramic alternant layer coatings were prepared by atmospheric plasma spraying and heat treated at 600, 700, 800, 900, and 1000?°C. The influence of heat treatment on microhardness, fracture toughness, and the structural evolution of the coatings on steel were investigated. Heat treatment promoted alternant layer interdiffusion within ceramic coatings, which could result a transformation from a lamellar morphology to mutual pinning. The interfacial diffusion between the bond coating and substrate was clearly demonstrated after heat treatment at different temperatures. Heat treatment also significantly affected the evolution of the hardness and fracture toughness. Temperature strongly affected the microhardness of the specimens, and the hardness arrived to the highest value at 1000?°C. The formation of a new Mg₂Al₆Ti₇O₂₅ phase and alternant layer mutual pinning were beneficial to hardness improvement, and heat treatment also significantly improved fracture toughness.