Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders

The oxidation behavior of hot-pressed Al₂O₃–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al₂O₃–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al₂O₃–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

Investigation on the preparation and machinability of the B4C/BN nanocomposites by hot-pressing process

The machinable B₄C/BN nanocomposites were fabricated by the hot-pressing process using the B₄C/BN nanocomposite powders at 1850 °C for 1 h under the pressure of 30 MPa. The nanocomposite powders with the microstructure of micro-sized B₄C particles coated with amorphous nano-sized BN particles were prepared by the chemical reaction of H₃BO₃ and CO(NH₂)₂ on the surface of B₄C particles at high temperature. Then the amorphous BN transformed into the hexagonal-BN (h-BN) after the hot-pressing process at 1850 °C. The microstructure investigations of the B₄C/BN nanocomposites sintered samples showed that the nano-sized h-BN particles were homogenously distributed within the matrix grains as well as at the matrix grains boundaries. With the increasing content of h-BN, the relative density of the B₄C/BN nanocomposites decreased gradually. The fracture strength and fracture toughness of the B₄C/BN nanocomposites decreased gradually, the fracture strength and fracture toughness of the B₄C/BN nanocomposites with the h-BN content of 10 wt.% and 20 wt.% achieved high values. The Vickers hardness of the B₄C/BN nanocomposites decreased remarkably with the increasing content of h-BN, while the drilling rates and machinability of the B₄C/BN nanocomposites increased significantly. The B₄C/BN nanocomposites with the h-BN content more than 20 wt.% exhibited excellent machinability.     

Effect of sintering temperature on the structure and properties of polycrystalline cubic boron nitride prepared by SPS

The polycrystalline cubic boron nitride (PcBN) with Si₃N₄–AlN–Al₂O₃–Y₂O₃ ceramic system as binding agents was prepared by spark plasma sintering (SPS). The starting materials Si₃N₄, AlN, Al₂O₃, Y₂O₃, and cBN in the ratio of 22:14:10:4:50 were heated to a sintering temperature between 1250 °C and 1450 °C at a heating rate of 300 °C/min, with a holding time of 5 min in nitrogen atmosphere. The microstructure, phase constitution, microhardness and fracture toughness of the prepared PcBN were then studied. It was shown that the Si₃N₄–AlN–Al₂O₃–Y₂O₃–cBN polycrystalline materials were densified in a very short sintering time resulting in materials with relative densities of more than 95%. When the sintering temperature increased, the microhardness and fracture toughness of prepared PcBN were also increased. The microhardness of PcBN prepared at 1250–1450 °C was between 28.0 ± 0.5 GPa and 48.0 ± 0.9 GPa, and its fracture toughness K_IC was from 7.5 ± 0.2 MPa m^1/2 to 11.5 ± 0.3 MPa m^1/2. Microstructure study showed that the ceramic-binding agents bonded with cubic boron nitride particles firmly. Our work demonstrated that spark plasma sintering technology could become a novel method for the preparation of PcBN cutting materials.     

Strengthening mechanisms in aluminum containing coherent Al3Sc precipitates and incoherent Al2O3 dispersoids

Dispersion-strengthened-cast aluminum (DSC-Al), consisting of a coarse-grained aluminum matrix containing two populations of particles (30 vol.% of 300 nm Al₂O₃ incoherent dispersoids and 0.2–0.3 vol.% of 6–60 nm coherent Al₃Sc precipitates), was studied. At ambient and elevated temperatures, both populations of particles contribute to strengthening. At 300 °C, creep threshold stresses are considerably higher than for control materials with a single population of either Al₂O₃ dispersoids or Al₃Sc precipitates. This synergistic effect is modeled by considering dislocations pinned at the departure side of incoherent Al₂O₃ dispersoids (detachment model) and simultaneously subjected to elastic interactions from neighboring coherent Al₃Sc precipitates.     

Al2O3/Mo composite and its tribological behavior against AISI201 stainless steel at elevated temperatures

Al₂O₃-reinforced molybdenum (Mo) composites were successfully prepared by powder metallurgy to improve the wear resistance of Mo components at high temperature. The reinforced Al₂O₃ particles are uniformly distributed in the Mo matrix; thus, the Al₂O₃/Mo composite is harder than monolithic Mo. The friction coefficients of both monolithic Mo and the Al₂O₃/Mo composite decrease by 37% and 42%, respectively, at 700 °C compared with those at room temperature (self-lubricating phenomenon). This phenomenon is attributed to the formation of very soft MoO₃ and FeMoO₄ metal oxides on the friction surface at high temperature. The Al₂O₃/Mo composite has better wear resistance than monolithic Mo at both room temperature and at 700 °C. The notable resistance of the composite particularly at 700 °C can be attributed to its increased hardness and the soft tribofilm forming on the worn surface.     

Unlubricated friction and wear behaviors of Al2O3/TiC ceramic cutting tool materials from high temperature tribological tests

The unlubricated friction and wear behaviors of Al₂O₃/TiC ceramic tool materials were evaluated in ambient air at temperature up to 800 °C by high temperature tribological tests. The friction coefficient and wear rates were measured. The microstructural changes and the wear surface features of the ceramics were examined by scanning electron microscopy. Results showed that the temperature had an important effect on the friction and wear behaviors of this Al₂O₃ based ceramic. The friction coefficient decreased with the increase of temperature, and the Al₂O₃/TiC ceramics exhibited the lowest friction coefficient in the case of 800 °C sliding operation. The wear rates increased with the increase of temperature. During sliding at temperature above 600 °C, oxidation of the TiC is to be expected, and the formation of lubricious oxide film on the wear track is beneficial to the reduction of friction coefficient. The wear mechanism of the composites at temperature less than 400 °C was primary abrasive wear, and the mechanisms of oxidative wear dominated in the case of 800 °C sliding operation.     

In-situ sintering reaction of Al2O3–LaAl11O18–ZrO2 composite

Al₂O₃–LaAl₁₁O₁₈–ZrO₂ composites were prepared by in situ sintering reaction of different proportions of Al₂O₃ and La₂Zr₂O₇. The studied batches were uniaxially pressed and pressureless sintered at 1600 °C up to 1725 °C for 1 h. Phase composition study reveals that the only present phases are alumina, lanthanum hexaluminates and zirconia. No other intermediate phases are present. Rodlike LaAl₁₁O₁₈ was observed in the sintered bodies containing more than 25 wt.% LaAl₁₁O₁₈. The effect of rodlike particles on the densification and mechanical behavior was discussed. It was found that increasing the LaAl₁₁O₁₈ content more than 25 wt.% enhances the fracture toughness, but reduces both the bending strength and the hardness of the sintered composites.     

Effect of Al addition on SiC–B4C cermet prepared by pressureless sintering and spark plasma sintering methods

SiC–B₄C–Al cermets containing 5, 10 and 20 wt.% of Al were fabricated by high-energy planetary milling followed by conventional sintering and spark plasma sintering (SPS) techniques separately. The average particle size reduced to ~ 3 μm from an initial size of 45 μm after 10 h of milling. The as-milled powders were conventionally sintered at 1950 °C for 30 min under argon atmosphere and SPS was carried out at 1300 °C for 5 min under 50 MPa applied pressure. The formation of Al₈B₄C₇ and AlB₁₂ phases during conventional sintering and SPS were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The formation of Al₈B₄C₇ at 700 °C and AlB₁₂ at 1000 °C was well supported by XRD and differential scanning calorimetry (DSC). The maximum relative density, microhardness and indentation fracture resistance of SiC–B₄C–10Al consolidated by SPS are 97%, 23.80 GPa and 3.28 MPa·m^1/2, respectively.     

Effect of the aluminum content on the mechanochemical behavior in ternary system Al–B2O3–C

The effect of aluminum content on the mechanochemical behavior of ternary system Al–B₂O₃–C to fabricate Al₂O₃/B₄C composite was investigated. A mixture of boron oxide powders along with different amounts of aluminum and graphite was activated in a ball mill. The value of Al content varied from 2 mol to 7 mol compared to the stoichiometric mole ratios (4 mol). Thermodynamics evaluation indicates that the value of Al content in the mixture plays a key role and overall reaction enthalpy and adiabatic temperature altered by variation of aluminum and carbon content. Experimental findings revealed that at low aluminum content (2 mol Al), aluminothermic reaction proceeded in gradual mode and no carbothermal reduction took place. Increase in Al content up to 3 mol led to a change in the mode of aluminothermic reaction to MSR (mechanically induced self-propagating reaction) and gradual occurrence of carbothermic reaction. By increasing the amount of Al (10/3–4 mol Al), aluminothermic reaction provided sufficient heat for activating endothermic carbothermic reduction; hence, both reducing reactions happened simultaneously. Further increase in Al content (7 mol Al) led to gradual aluminothermic reaction and excess Al acted as inert matrix.     

Effect of Al2O3 ceramic binder on mechanical and microstructure properties of spark plasma sintered WC-Co cermets

This study investigates how the partial replacement of Co with Al₂O₃ ceramic binder has an effect on the sintering behaviour, microstructure, and final mechanical properties of WC-Co cermets via spark plasma sintering. To examine this, three batches (WC-6 wt%Co, WC-3 wt%Co-3 wt%Al₂O_3, and WC-6 wt%Al₂O₃) were mixed through high energy ball mill, and sintering was carried out at temperatures of 1350 °C and 1600 °C. The results showed nearly full dense WC-Co cermets at different temperatures. It was shown that WC-6 wt%Al₂O₃, in comparison to reference WC-6 wt%Co cermet, not only led to the rise in sintering temperature from 1350 °C to 1600 °C, but also reduced its strength and toughness. But replacing some part of Co with alumina (WC-3 wt%Co-3 wt%Al₂O₃) exhibited the combination of high strength (1095 MPa), hardness (17.62 GPa), and fracture toughness (19.46 MPa·m^1/2).     

Effect of reinforcement NbC phase on the mechanical properties of Al2O3-NbC nanocomposites obtained by spark plasma sintering

The sintering behavior of Al₂O₃-NbC nanocomposites fabricated via conventional and spark plasma sintering (SPS) was investigated. The nanometric powders of NbC were prepared by reactive high-energy milling, deagglomerated, leached with acid, added to the Al₂O₃ matrix in the proportion of 5 vol% and dried under airflow. Then, the nanocomposite powders were densified at different temperatures, 1450–1600 °C. Effect of sintering temperature on the microstructure and mechanical properties such as hardness, toughness and bending strength were analyzed. The Al₂O₃-NbC nanocomposites obtained by SPS show full density and maximum hardness value > 25 GPa and bending strength of 532 MPa at 1500 °C. Microstructure observations indicate that NbC nanoparticles are dispersed homogeneously within Al₂O₃ matrix and limit their grain growth. Scanning electron microscopy examination of the fracture surfaces of dense samples obtained at 1600 °C by SPS revealed partial melting of the particle surfaces due to the discharge effect.     

The effect of Fe2Al5 as reducing agent in intermediate steps of 'Al2O3/TiC-Fe composite production process

Al₂O₃/TiC-Fe cutting tools can be produced in-situ by using ilmenite, aluminum and graphite. The formation mechanism of the system was investigated in some researches. Most of them have diagnosed the aluminum as reducing agent of TiO₂, but in this study, it is proven that Fe₂Al₅ acts as the reducing agent for TiO₂ in the intermediate steps of the reactions. To achieve this goal, firstly, the reactions of synthesized ilmenite, aluminum and graphite system were investigated in detail. Then, pure Fe₂Al₅ was synthesized, and mixed with definite amount of TiO₂. The sample mixture was heat treated at the same temperature at the real reaction temperature, namely, 930 °C. The final products were analyzed with XRD. It was found that the product heat treatment of F₂Al₅ and TiO₂ mixture was the same as the sample of ilmenite, aluminum and graphite mixture with a molar ratio of 1:2:1 heat treated at 930 °C.     

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

Microstructural development during high-temperature oxidation of Ti₂AlC below 1300 °C involves gradual formation of an outer discontinuous TiO₂ layer and an inner dense and continuous α-Al₂O₃ layer. After heating at 1400 °C, an outer layer of mixed TiO₂ and Al₂TiO₅ phases and a cracked α-Al₂O₃ inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO₂ grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO₂ grains due to thermal expansion mismatch of the phases (TiO₂, α-Al₂O₃ and Al₂TiO₅) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO₂ grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al₂TiO₅ and the volume changes associated with the reactions during Ti₂AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O^2? and outward diffusion of Al³⁺ and Ti⁴⁺. The weakly bound Al leaves the Al atom plane in the layered structure of Ti₂AlC, and diffuses outward to form a protective inner α-Al₂O₃ layer between 1100 and 1300 °C. However, the α-Al₂O₃ layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.     

Synthesis,crystal structure and thermal behavior of Na4[B10O16(OH)2]·4H2O

New hydrated sodium borate Na₄[B₁₀O₁₆(OH)₂]·4H₂O has been synthesized under mild hydrothermal conditions at 170 °C. The structure was determined by single-crystal X-ray diffraction and further characterized by FT-IR, Raman spectra and DTA-TG. It crystallizes in the monoclinic space group Pc with a unit cell of dimension a = 11.323(2) Å, b = 6.5621(14) Å, c = 12.244(3) Å, α = 90°, β = 91.050(3)°, γ = 90°, V = 909.7(3) Å³, Z = 2. The crystal structure of Na₄[B₁₀O₁₆(OH)₂]·4H₂O consists of Na–O polyhedra and [B₁₀O₁₆(OH)₂]⁴⁻ polyborate anions. Dehydration of this compound occurs in three steps and leads to an amorphous phase which undergoes crystallization.     

Wear and friction of TiAlN/VN coatings against Al2O3 in air at room and elevated temperatures

TiAlN/VN multilayer coatings exhibit excellent dry sliding wear resistance and low friction coefficient, reported to be associated with the formation of self-lubricating V₂O₅. To investigate this hypothesis, dry sliding ball-on-disc wear tests of TiAlN/VN coatings on flat stainless steel substrates were undertaken against Al₂O₃ at 25 °C, 300 °C and 635 °C in air. The coating exhibited increased wear rate with temperature. The friction coefficient was 0.53 at 25 °C, which increased to 1.03 at 300 °C and decreased to 0.46 at 635 °C. Detailed investigation of the worn surfaces was undertaken using site-specific transmission electron microscopy (TEM) via focused ion beam (FIB) microscopy, along with Fourier transform infrared (FTIR) and Raman spectroscopy. Microstructure and tribo-induced chemical reactions at these temperatures were correlated with the coating’s wear and friction behaviour. The friction behaviour at room temperature is attributed to the presence of a thin hydrated tribofilm and the presence of V₂O₅ at high temperature.     

High temperature oxidation of AlCrFe complex metallic alloys

Isothermal oxidation of Al₆₅Cr₂₇Fe₈ and Al₈₀Cr₁₅Fe₅ was studied in the 600–1080 °C range. Formation of transient alumina layers is obtained up to 900 °C. On Al₆₅Cr₂₇Fe₈ transient to α-phase transformations occur when performing oxidation at 1000 °C, together with the possible appearance of (Al_0.9Cr_0.1)₂O₃. At 1080 °C, direct formation of α-alumina is obtained. On Al₈₀Cr₁₅Fe₅, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900 °C, revealing thermal etching of the underneath alloy surface. At 1050 °C the α-Al₂O₃ scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.     

The ternary Al–Ce–Cu phase diagram in the aluminum-rich corner

The phase diagram of the Al–Ce–Cu system in the aluminum-rich corner (concentrations up to 35 wt.% Cu and up to 16 wt.% Ce) was studied. Phases and invariant reactions were identified. Liquidus and solidus surfaces, three isothermal cross-sections (540, 590 and 200 °C) and five vertical cross-sections were constructed. It was shown that Al₄Ce (Al₁₁Ce₃), Al₈CeCu₄ and Al₂Cu were in equilibrium with the aluminum solid solution (Al). It was determined that about 17 wt.% Cu dissolves in Al₄Ce; the existence of the Al₄CeCu phase was not confirmed. Experimentally determined parameters of the invariant quasi-binary eutectic reaction L → (Al) + Al₈CeCu₄ (610 °C, 14 wt.% Cu and 7 wt.% Ce) were essentially different from those published earlier. This eutectic had a fine microstructure and was stable to coarsening (after fragmentation and spheroidizing) during the heating process even though the average particle size did not grow larger than 2 μm after annealing at 590 °C.     

Microstructure evolution and densification kinetics of Al2O3/Ti(C,N) ceramic tool material by microwave sintering

The microstructure evolution and densification kinetics of Al₂O₃/Ti(C,N) ceramic tool material during microwave sintering were studied. The density and grain growth significantly increases at the temperatures higher than 1400 °C. The calculated kinetics parameter n indicates that volume diffusion is the main densification mechanism when the sintering temperature is below 1300 °C, while grain boundary diffusion plays a leading role in the densification process when the sintering temperature is higher than 1300 °C. The grain growth activation energy of Al₂O₃/Ti(C,N) composite is 48.82 KJ/mol, which is much lower than those of monolithic Al₂O₃ in the microwave sintering and conventional sintering. The results suggested that the Al₂O₃/Ti(C,N) ceramic tool material with nearly full densification and fine grains can be prepared by two-step microwave sintering.     

Mechanical and barrier properties of MOCVD processed alumina coatings on Ti6Al4V titanium alloy

This study focuses on the implementation of different aluminum oxide coatings processed by metal-organic chemical vapor deposition from aluminum tri-isopropoxide on commercial Ti6Al4V titanium alloy to improve its high temperature corrosion resistance. Films grown at 350 °C and at 480 °C are amorphous and correspond to formulas AlOOH, and Al₂O₃, respectively. Those deposited at 700 °C are composed of γ-Al₂O₃ nanocrystals dispersed in a matrix of amorphous alumina. Their mechanical properties and adhesion to the substrates were investigated by indentation, scratch and micro tensile tests. Hardness and rigidity of the films increase with increasing deposition temperature. The hardness of the coatings prepared at 350 °C and 480 °C is 5.8 ± 0.7 GPa and 10.8 ± 0.8 GPa respectively. Their Young's modulus is 92 ± 8 GPa (350 °C) and 155 ± 6 GPa (480 °C). Scratch tests cause adhesive failures of the films grown at 350 °C and 480 °C whereas cohesive failure is observed for the nanocrystalline one, grown at 700 °C. Micro tensile tests show a more progressive cracking of the latter films than on the amorphous ones. The films allow maintaining good mechanical properties after corrosion with NaCl deposit during 100 h at 450 °C. After corrosion test only the film deposited at 700 °C yields an elongation at break comparable to that of the as processed samples without corrosion. The as established processing–structure–properties relation paves the way to engineer MOCVD aluminum oxide complex coatings which meet the specifications of the high temperature corrosion protection of titanium alloys with regard to the targeted applications.     

NiAl–Al2O3 composites produced by pulse plasma sintering with the participation of the SHS reaction

The paper presents the results of examinations of the NiAl–Al₂O₃ sinters (13, 38 and 55 vol% of Al₂O₃) produced from a mixture of nickel, aluminum and alumina powders in a single technological process, using the pulse plasma sintering (PPS) method. By subjecting the elemental powders to a PPS process for 900 s, we obtained NiAl–Al₂O₃ composites of a hardness ranging from 480 HV10 (13% Al₂O₃) to 680 HV10 (55% Al₂O₃). The fracture toughness of the sintered materials depended on the amount of the dispersed Al₂O₃ phase. When examined with a Vickers indenter under a load of 10 kg, the composite containing 13% of Al₂O₃ showed no cracking. In the composites with 38% Al₂O₃ and 55% Al₂O₃ contents, the value of K_IC was 9.1 and 8.2 MPam^1/2, respectively.