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.     

Development of in situ NiAl–Al2O3 nanocomposite by reactive milling and spark plasma sintering

NiAl–10 vol.% Al₂O₃ in situ nanocomposite has been synthesized by reactive milling and subsequent spark plasma sintering. The synthesized nanocomposites have ～96% of theoretical density after sintering at 1000 °C for 5 min. Microstructural analysis of consolidated samples using TEM has revealed the presence of α-Al₂O₃ particles of 10–12 nm size in NiAl matrix of submicron grain size. Consolidated NiAl–10 vol.% Al₂O₃ nanocomposite has shown very high hardness of 772 HV_0.3 and compressive strength of 2456 MPa with ～14% plastic strain. The high hardness and compressive yield strength are attributed to the presence of nanocrystalline α-Al₂O₃ particles and the appreciable plastic strain is attributed to the submicron grains of NiAl.     

Preparation and characterization of Mo/Al2O3 composites

In order to improve the performance of molybdenum, the Mo/Al₂O₃ composites were prepared by using a hydrothermal method for the synthesis of the precursor powders and subsequent powder metallurgical processing. The morphologies of the composite powders and the microstructures and properties of the composites were investigated. Compared with the pure Mo powder, the grains of composite powders are smaller because of the existence of the fine Al₂O₃ particles. The results from the sintered composites show that the fine Al₂O₃ particles are evenly distributed in the Mo matrix and well bonded with the Mo matrix. With increasing Al₂O₃ content, all the values of the micro-hardness, compressive strength and flow stress at 0.08 strain are increased. The strengthening effect is more remarkable at elevated temperatures. At room temperature, the compressive strength and the flow stress at 0.08 strain of the composite with 40 vol.% Al₂O₃ are 1.67 and 2.01 times greater than those of pure molybdenum, respectively, while the values are up to 2.02 and 2.52 at 1100 °C.     

In-depth analysis of residual stress in an alumina coating on silicon nitride substrate using confocal Raman piezo-spectroscopy

Confocal Raman piezo-spectroscopy was applied to the non-destructive evaluation of residual stresses as they develop in a chemical-vapor-deposited Al₂O₃ coating on a Si₃N₄ ceramic substrate. According to a selected confocal configuration of the optical probe, with its focal plane set to in-depth scan the sample, the residual stress could be measured at various depths along the thickness of both coating and substrate. The residual stresses stored in the Al₂O₃ coating layer were measured using both the Cr³⁺ fluorescence band, located at 14,400 cm⁻¹ (R₁), and the Al₂O₃ Raman band at 417 cm⁻¹. When the R₁ fluorescence band was used, no variation could be resolved for the residual stress along the coating depth direction; in contrast, a clear in-depth stress distribution was observed when the 417 cm⁻¹ Raman band was used. The minimum stress magnitude was located at the coating external surface and the maximum at the coating/substrate interface. Given the high transparency of the Al₂O₃ coating, the residual stress field stored within the Si₃N₄ substrate could also be measured as a function of depth (according to the piezo-spectroscopic shift of the 206 cm⁻¹ Raman band of Si₃N₄). A deconvolution procedure of confocal spectra was proposed, which is based on the knowledge of the probe response functions of both coating and substrate materials. Laser probe deconvolution enabled us to retrieve the actual in-depth stress distribution from the stress distribution experimentally observed by defocusing experiments.     

Microindentation fracture behavior of surface modified alumina ceramic using glass infiltration

This study investigates the effect of a low expansion glass (Mg₃Al₂Si₆O₁₈) treatment on the surface fracture toughness of sintered alumina. The surface fracture toughness was determined by direct indentation method (Vickers indentations), carried out at different loads ranging from 9.8 to 196 N. The crack lengths on the surface at each load were found to be decreased (8–12%) by glass treatment and the corresponding crack resistance values increased by about 17–20%. Both sintered and glass treated specimens showed rising trend in crack resistance values as the indentation load was increased. There was also a significant increase in the Weibull modulus value of crack resistance. Improved properties of glass treated sample were attributed to the formation of a relatively larger process zone surrounding the crack, crack arrest behavior due to the compressive stresses and the crack bridging phenomena. The compressive stresses were generated from the thermo-elastic properties mismatch: (a) between the glass and the ceramic in the glass infiltrated zone, and (b) the glass–ceramic composite layer and the ceramic substrate.     

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.     

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.     

Analysis of residual thermal stresses in MoSi2 based laminated composites

The residual thermal stresses of MoSi₂-SiC_p + refractory metal foil laminated composites are estimated, in this paper, by both finite element methods (FEMs) as well as analytical expressions. It is observed in the present study that the refractory metal foils are found to possess compressive stresses, while the MoSi₂-SiC_p matrix layer is under tensile stresses. The Nb foil reinforced laminated composite is found to possess the lowest residual stresses owing to a lower thermal expansion mismatch between MoSi₂-SiC_p matrix and the Nb foil as compared to the Mo and Ta foils.     

Reactive synthesis of alumina-boron nitride composites

The reactions of aluminum borates (9Al₂O₃ · 2B₂O₃ and 2Al₂O₃ · B₂O₃) with aluminum nitride (AlN) have been used as a new chemical route to synthesize alumina-boron nitride (Al₂O₃–BN) composites. Reaction mechanisms were investigated by TG-DTA and static reaction process. The reactions started at around 1200 °C and completed at around 1500 °C. Soaking at temperatures higher than 1800 °C resulted in the reverse reaction that caused great weight loss. Hot pressing promoted the reactions due to the improved diffusion process. The in situ formed BN phase was in agglomerate shape located at the pockets of Al₂O₃ matrix particles and this distribution was suggested to be beneficial to the strength of materials with weak phase dispersoids. The fracture surface analysis demonstrated that the main fracture mode was transgranular, indicating the existence of a strong Al₂O₃ network in the in situ synthesized composites. The prepared composites exhibited high strength, low Young’s modulus and high strain tolerance.     

Mechanical behavior of 3Al2O3·2SiO2 films under nanoindentation

A complete and systematic nanoindentation study was conducted on a mullite (3Al₂O₃·2SiO₂) coating ～1 μm thick, deposited by means of chemical vapor deposition on a silicon carbide (SiC) substrate. The investigation included using different indenter tip geometries (Berkovich, spherical and cube-corner), complemented with atomic force microscopy and three-dimensional focused ion beam tomography to characterize the indentation response, deformation and damage micromechanisms. The intrinsic mechanical properties of the 3Al₂O₃·2SiO₂ film and the interfacial toughness of the coated (3Al₂O₃·2SiO₂/SiC) system were critically evaluated to assess the influence of substrate and film residual stresses. Through appropriate implementation of specific indenter tip geometry, different length-scale mechanical properties in the materials studied were successfully determined: yield strength and fracture toughness for the film, together with energy of adhesion per unit area and interface fracture toughness for the coated system.     

The influence of buffer/capping-layer-mediated stress on the coercivity of NdFeB films

The influence of buffer/capping-layer-mediated stress on the coercivity of NdFeB films is demonstrated. NdFeB films, 5 μm thick and rich in Nd, were deposited on Si/SiO₂ and Al₂O₃ substrates, with or without buffer/capping layers of Ta. The coercivity of Ta-free samples (～0.5 T) is significantly less than that achieved in samples with Ta present as a buffer and/or capping layer (1.7–1.8 T). The as-sputtered Ta layers are under strong compressive stress due to peening. During post-deposition annealing to crystallize the Nd₂Fe₁₄B phase, stresses are relieved in the Ta layer. This leads in turn to a compressive stress in the NdFeB layer, inducing extrusion of a Nd-rich phase up through the NdFeB layer. The high values of coercivity achieved in Ta-containing structures are attributed to good coverage of individual Nd₂Fe₁₄B grains with the redistributed Nd-rich phase.     

Insights into high temperature oxidation of Al2O3-forming Ti3AlC2

Insights into high temperature oxidation of Al₂O₃-forming Ti₃AlC₂ are made to understand the sub-parabolic oxidation rate and the absence of Al-depletion zone beneath the protective Al₂O₃ scale. The scale results from selective oxidation of Al that migrated from Ti₃AlC₂ according to Ti₃AlC₂ + 3/2y O₂ → Ti₃Al_1−yC₂ + 1/2y Al₂O₃, leaving Al vacancy in the substrate. Inward diffusion of oxygen via grain boundaries of the Al₂O₃ scale predominates for the scale growth. The Al₂O₃ grains grow with time, yielding sub-parabolic oxidation kinetics. The Al diffuses in a fast manner, accounting for the absence of Al-depletion zone.     

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.     

Sintering of Al2O3–TiB2 nano-composite derived from milling assisted sol–gel method

Synthesis and sintering of an alumina /titanium diboride nano-composite have been studied as an alternative for pure titanium diboride for ceramic armor applications. Addition of TiB₂ particles to an Al₂O₃ matrix can improve its fracture toughness, hardness and flexural strength and offer advantages with respect to wear and fracture behavior. This contribution, for the first time, reports the sintering, microstructure, and properties of Al₂O₃–TiB₂ nano-composite densified with no sintering aids. Nano-composite powder was produced by combination of sol–gel and mechano-chemical methods. The densification experiments were carried out using both hot pressing and pressureless sintering routes. In the pressureless sintering route, a maximum of 92.3% of the theoretical density was achieved after sintering at 1850 °C for 2 h under vacuum. However, hot pressing at 1500 °C for 2 h under the same condition led to achieving a 99% of the theoretical density. The hot pressed Al₂O₃–TiB₂ nano-composites exhibit high Vickers hardness (16.1 GPa) and a modest indentation toughness (~ 4.2 MPa.m^1/2).     

Wear mechanism of CrN/NbN superlattice coating sliding against various counterbodies

Wear properties of CrN/NbN superlattice coating deposited on the WC-12Co substrate was investigated while using 100Cr6 steel, SiC and Al₂O₃ ball as counterbodies for friction pairs. The value of friction coefficient and wear rate was lowest at ~ 0.01 and 2.6 × 10^− 7 mm³/Nm, respectively, when coating slides against Al₂O₃ ball. In contrast, friction coefficient and wear rate were increased while sliding with steel and SiC ball. The deviation in friction coefficient was described by mechanical and chemical properties of these balls. Hardness of Al₂O₃ and SiC ball was comparable but significant deviation in friction coefficient was observed. That is related to oxidation resistance of these balls which is high for Al₂O₃ compared to SiC ball as evident by Raman analysis of the wear track. However, hardness and oxidation resistance were low for steel ball which shows oxidational wear mechanism.     

Splat morphology of plasma sprayed aluminum oxide reinforced with carbon nanotubes: A comparison between experiments and simulation

This study elucidates the effect of carbon nanotube (CNT) addition on the splat formation in plasma sprayed aluminum oxide (Al₂O₃) composite coating using experimental and computational methods. CNT content was varied as 0, 4 and 8 wt.% in Al₂O₃ matrix. With an increasing CNT content, splat morphology became more circular and disk-shaped. The average diameter of disk-shaped splats increased from 28.6 ± 1.4 μm for Al₂O₃ to 43.2 ± 1.3 μm for Al₂O₃–8 wt.% CNT. The population density of splats with fingers, fragments, and voids was the lowest for the highest (8 wt.%) CNT content. The addition of CNTs resulted in two simultaneously competing phenomena viz. increased thermal capacity and increased viscosity of the melt. Increased thermal capacity delayed the localized solidification resulting in higher splat diameter while agglomeration of CNTs at the periphery of the splat results in higher viscosity of the melt which suppresses the splat fragmentation that leads to increased population density of disk shaped splats. Splat morphology of three compositions was also simulated using SIMDROP software, which showed a good agreement with the experimentally collected splats.     

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.     

Extruded ZrSiO4 particulate-reinforced LZSA glass–ceramics matrix composite

This work reports on the characterization of ZrSiO₄ particulate-reinforced Li₂O–ZrO₂–SiO₂–Al₂O₃ (LZSA) glass–ceramic matrix composite added with bentonite as binder and formed by extrusion. The glass batches and composites were characterized on the point of view of their typical physical/mechanical and chemical properties. Composition with 60 wt.% ZrSiO₄ was preliminary selected, since it showed the best results in terms of bending strength (190 MPa) and deep abrasion resistance (51 mm³). The same composition as before but added with 7 wt.% bentonite was selected for further studies since it exhibited the highest plasticity index, which resulted in good billets after extrusion. In this last case, the extruded samples, after sintering at 1150 °C for 10 min, showed a thermal linear shrinkage of 14% and deep abrasion resistance and bending strength of 51 mm³ and 220 MPa, respectively.     

The influence of the grain boundary phase on the mechanical properties of Si3N4–MoSi2 composites

The microstructure and mechanical properties of Si₃N₄–MoSi₂ composites doped with two different sintering additive systems, Y₂O₃–Al₂O₃ and Lu₂O₃, were investigated. It was found that the composite doped with Y₂O₃–Al₂O₃ had an amorphous grain boundary phase, while the grain boundary phase of the Lu₂O₃-doped composites was completely crystallized. The Si₃N₄–MoSi₂ composite containing Lu₂O₃ had higher elastic modulus and better creep resistance at elevated temperatures (>1000 °C) than the composite doped with Y₂O₃–Al₂O₃. This is attributed to the crystallization and higher softening temperature of the Lu₂O₃-doped grain boundary phase compared with that doped with Y₂O₃–Al₂O₃. However, the toughness and strength were not influenced significantly by the grain boundary phase. The inclusion of MoSi₂ particles in Si₃N₄ can improve their fracture toughness through residual stresses induced by the coefficient of thermal expansion mismatch of Si₃N₄ and MoSi₂. The strength decreased significantly at temperatures over 1000 °C due to the brittle–ductile transition of the MoSi₂ phase.     

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.