Oxidation resistance of Ni-toughened Al2O3

Adding nickel inclusions into alumina can enhance its strength and toughness. However, the oxidation resistance of alumina is degraded due to the presence of metallic nickel. In the present study, the oxidation kinetics of Ni-toughened Al₂O₃ in the temperature region from 1000 to 1300 °C are investigated. In the Al₂O₃/Ni composites, the Ni inclusions are isolated to each other within the Al₂O₃ matrix as the Ni content is less than 15 vol.%. The oxidation of the composites is mainly a diffusional process, nickel ions diffuse out and oxygen ions diffuse in. A dense NiAl₂O₄ spinel is formed on the surface of the composite after oxidation. The oxidation rate constants of the alumina incorporated with isolated Ni inclusions are in a comparable range with those of hot-pressed silicon nitride.     

The influence of carbon on the microstructure and wear resistance of alumina

The influence of carbon as a dopant on grain growth and wear resistance of polycrystalline alumina was evaluated. Carbon was introduced into alumina by sintering in a carbon-rich environment (graphite furnace under flowing He), and/or by residual carbon from organic binders used during the green body consolidation process. Samples were sintered at 1600°C for 2 h. Doping alumina with carbon resulted in a reduced grain size after sintering, correlated to solute-drag, and graphite particle-drag for high concentrations of carbon (~3 wt.%). The material response to abrasive wear was quantified by measuring the sample area cut for a defined time using a diamond wafering saw, as a function of grain size and carbon content. Sintering alumina with carbon resulted in a significant increase in wear resistance, as a result of the reduced grain size.     

Abrasive wear of Al2O3–SiC and Al2O3–(SiC)–C composites with micrometer- and submicrometer-sized alumina matrix grains

The response of Al₂O₃, Al₂O₃–SiC–(C) and Al₂O₃–C nanocomposites to grinding was investigated in terms of changes of quality of ground surfaces and of the weight losses with time. The study used monolithic polycrystalline aluminas as references, and alumina-based composites with nanosized SiC and C inclusions and with alumina matrix grain size varying from submicrometer to approximately 4 μm. The studied materials can be roughly divided into two groups. Materials with submicrometer alumina matrix grains (Group 1) wear predominantly by plastic deformation and grooving. Coarse-grained materials (Group 2) wear by mixed wear mechanism involving crack initiation and interlinking accompanied by grain pull-out, plastic deformation and grooving. The wear rate of composites increases with increasing volume fraction of SiC. The Group 2 materials wear much faster then those with submicron microstructure. In all cases (with one exception) the wear resistance of composites was higher than that of pure aluminas of comparable grain sizes used as reference materials.     

Processing and characterization of fiber-reinforced and layered alumina - graphene composites

The aim of the present contribution is the processing and characterization of fiber-reinforced and layered alumina - graphene composites, prepared by the combination of electrospinning, calcination, chemical vapor deposition (CVD) and spark plasma sintering (SPS). The fiber-reinforced composite contains homogenously distributed graphene-coated polycrystalline alumina microfibers in the Al₂O₃ matrix. The layered composites contain Al₂O₃ layers and layers of graphene-coated alumina microfibers or layers of graphene-coated alumina grains of submicron size. The systems with high density, 99.5–99.9 %, show different grain sizes of Al₂O₃ in their constituents, changing from 0.08 to 1.9 μm in comparison to the monolithic alumina with the average grain size of 2.6 μm. The composites and their layers show increased electrical conductivity, hardness, and fracture toughness by approximately five orders of magnitude, 31 %, and 8%, respectively, in comparison to the monolithic alumina due to the presence of graphene layers, small grain-sized alumina, and microfibers in the composites.     

Influence of the SiC grain size on the wear behaviour of Al2O3/SiC composites

Dry ceramic block-on-steel ring wear tests were performed at high loads in several Al₂O₃/20 vol.% SiC composites as a function of the SiC grain size, which ranged from 0.2 to 4.5 μm in d₅₀. The wear resistance of the monolithic alumina was radically improved by the addition of the SiC particles, reducing down to one order of magnitude wear rate. Two different behaviours were identified according to the microstructural observations on the worm surfaces: intergranular fracture and grain pull-out in the monolithic Al₂O₃, and plastic deformation and surface polishing in the composites. The wear resistance of the Al₂O₃/SiC composites increased with the SiC grain size due to their fracture toughness enhancement.     

Processing of alumina grinding media using scandia as a sintering aid

Wear resistance is one of the essential properties of grinding media. Good wear resistance is the quality guarantee of preparing high-purity ultrafine powder. This paper mainly studied the influence rule and enhanced mechanism of Sc₂O₃ on the wear resistance of Al₂O₃ ceramics. The relationship between Sc₂O₃ content and sintering properties, wear resistance, and microstructure of ceramics was studied. According to research findings, Sc₂O₃ could promote the densification of ceramics, improve the morphology of grain boundaries, enhance the binding force of grains, and increase the mechanical property of alumina grain.     

Sliding wear behaviour of alumina/nickel nanocomposites processed by a conventional sintering route

The wear resistance of Al₂O₃/2.5 vol.% Ni nanocomposites sintered by a conventional route was studied under ball-on-disk dry sliding conditions and compared with the same nanocomposites but consolidated by spark plasma sintering, together with alumina obtained by the same technique and by hot pressing. The results showed an improvement of about 0.5, 1 and 2 orders of magnitude, respectively. Thus, alumina/Ni nanocomposites processed by conventional route can compete, in cost and wear performance, with nanomaterials obtained by more sophisticated techniques.     

Effect of nano CuO addition on the tribo-mechanical behavior of alumina ceramics in non-conformal contact

Sintering of alumina from 1500°C to 1650°C and tribo-mechanical properties at room temperature had been investigated using nano CuO as a sintering aid. Bulk density gradually increases with sintering temperature from 1500°C to 1600°C and is optimized at 1600°C, beyond this, bulk density does not significantly increase at 1650°C. The addition of 2 wt% CuO showed the best result on densification. Densification of about 97.74% was attained at 1600°C with the incorporation of 2 wt% CuO. Nano CuO at grain boundaries forms CuAl₂O₄ liquid which modifies the morphology of the grain and improves mechanical properties. The formation of self-lubricating tribo-film on the wear track results in a low coefficient of friction <0.2 and reduces specific wear rate. 4 wt% CuO addition increases contact tensile stress (σ_max) by 51.2% and high Hertzian contact pressure (P_max≈1.51 GPa) causes plastic deformation of wear track. The re-solidified strengthening bond phase on the wear track simultaneously increases in friction coefficient and wear resistance with CuO addition. The optimizing effect of CuO addition shows that 2 wt% significantly decreases wear rate, and increases hardness and fracture toughness.     

Effect of grain size and amount of zirconia on the physical and mechanical properties and the wear resistance of zirconia-toughened alumina

The effects of the ZrO₂ content and the particle size of ZrO₂ powders on the microstructure, phase composition, physical and mechanical properties, and the abrasion wear resistance of advanced Al₂O₃ ceramics and zirconia-toughened alumina (ZTA) composites containing 0 to 30 mass% yttria-stabilised zirconia (YSZ) were investigated. The composite with a ZTA content of 30 mass% of ZrO₂ exhibited the greatest resistance to abrasion wear. α-Al₂O₃ reflex broadening (hkl = 113) as a result of the microstresses in the Al₂O₃ crystal lattice during sandblasting decreased with increasing ZrO₂ amount, where the ZrO₂ particles located along the grain boundaries of Al₂O₃, hindering their growth and deformation. The use of nanodispersed ZrO₂ powder produced by the plasma chemical technique led to a 1.5-fold increase in wear resistance in the resultant ZTA ceramic.     

Mechanical properties and sliding wear behaviour of Al2O3-SiC nanocomposites with 3–20 vol% SiC

Al₂O₃/SiC composites containing different volume fractions (3, 5, 10, 15, and 20 vol%) of SiC particles were produced by conventional mixing of alumina and silicon carbide powders, followed by hot pressing at 1740 °C for 1 h under the pressure of 30 MPa in the atmosphere of Ar. The influence of the volume fraction and size of SiC particles (two different powders with the mean size of SiC particles 40 and 200 nm were used), and final microstructure on mechanical properties and dry sliding wear behaviour in ball-on-disc arrangement were evaluated. The properties of the composites were related to a monolithic Al₂O₃ reference. Microstructure of the composites was significantly affected by the volume fraction of added SiC, with the mean size of alumina matrix grains decreasing with increasing content of SiC particles. The addition of SiC moderately improved the Vickers hardness. Fracture toughness was lower with respect to monolithic Al₂O₃, irrespective of the volume fraction and size of SiC particles. Al₂O₃/SiC nanocomposites conferred significant benefits in terms of wear behaviour under the conditions of mild dry sliding wear. Wear resistance of the alumina reference was poor, especially at the applied load of 50 N. The wear rates of composites markedly decreased with increasing volume fraction of SiC. Wear of the composites was also influenced by the material of counterparts, especially their hardness, with softer counterparts resulting in lower wear rates. All composites wore by a combination of grain pull-out with plastic deformation associated with grooving and small contribution of mechanical wear (micro-fracture). No influence of SiC particle size on wear rate or mechanism of wear was observed in the materials with identical volume fractions of SiC.     

The influence of additives on microstrucutre of sub-micron alumina ceramics prepared by two-stage sintering

For various systems two-stage sintering has been reported as a successful way of suppressing the grain growth in the final stage of densification of polycrystalline ceramics. Our previous results on two-stage sintering of high purity submicrometre polycrystalline alumina indicate limited efficiency of the process with respect to suppression of grain growth. The present work deals with the influence of deliberate additions of various metal oxides (500 ppm of MgO, Y₂O₃ or ZrO₂) whose grain growth retarding effect in conventional sintering has been well documented, on two-stage sintering of submicrometre alumina ceramics. The addition of MgO was observed to enhance densification. Addition of yttria and zirconia impaired densification, but addition of all three dopants resulted in suppression of the grain growth and microstructure refinement in comparison to undoped alumina.     

Ceramic matrix composites in the alumina/5-30 vol.% YAG system

The preparation technique of the particulate composite materials in the alumina/YAG system was elaborated. Within alumina particles suspension yttria precursor was precipitated with ammonium carbonate. Drying and calcination at 600 °C resulted in the mixture of alumina and yttria particles, the latter being much finer than alumina particles. This mixture was additionally homogenized by short attrition milling in an aqueous suspension. Sintering of such powders results in the materials composed of YAG inclusions of sizes smaller than shown by alumina grains and evenly distributed within the matrix. YAG particles result from the reaction of Y₂O₃ with Al₂O₃ during heat treatment. YAG inclusions limit effectively grain growth of the alumina matrix. Hardness, fracture toughness, strength, Young modulus and wear susceptibility of composites and pure alumina were measured. Composites show higher hardness and in some cases higher fracture toughness and wear resistance than pure alumina polycrystals.     

The influence of MgO,Y2O3 and ZrO2 additions on densification and grain growth of submicrometre alumina sintered by SPS and HIP

Spark plasma sintering followed by hot isostatic pressing was applied for preparation of polycrystalline alumina with submicron grain size. The effect of additives known to influence both densification and grain growth of alumina, such as MgO, ZrO₂ and Y₂O₃ on microstructure development was studied. In the reference undoped alumina the SPS resulted in some microstructure refinement in comparison to conventionally sintered materials. Relative density >99% was achieved at temperatures >1200 °C, but high temperatures led to rapid grain growth. Addition of 500 ppm of MgO, ZrO₂ and Y₂O₃ led, under the same sintering conditions, to microstructure refinement, but inhibited densification. Doped materials with mean grain size <400 nm were prepared, but the relative density did not exceed 97.9%. Subsequent hot isostatic pressing (HIP) at 1200 and 1250 °C led to quick attainment of full density followed by rapid grain growth. The temperature of 1250 °C was required for complete densification of Y₂O₃ and ZrO₂-doped polycrystalline alumina by HIP (relative density >99.8%), and resulted in fully dense opaque materials with mean grain size<500 nm.     

Effect of temperature and holding time on the densification of alumina obtained by two-step sintering

The two-step sintering technique is a process of controlling the sintering curve, which provides materials with higher density and smaller grain size when compared to conventional sintering. This technique was evaluated by optical dilatometry with three commercial alumina powders of different purity (92, 96 and 99 wt% of Al₂O₃) and particle size (between 0.73 and 2.16 µm). Different sintering conditions in the first (temperature, T1) and second (temperature, T2, and holding time, t2) steps were studied in order to evaluate the effect of these variables on densification and grain growth. Considering T1 as the temperature at which a relative density (D_rel) of 83% was achieved, and for the range of conditions tested, it was found that higher D_rel values and lower grain size of alumina were obtained with higher T2 and lower t2. Alumina with 99 wt% purity sintered at T1 of 1550 °C for 5 min and T2 of 1500 °C for 4 h showed the best relationship between higher densification (~96% relative density) and reduced grain size (0.94±0.15 µm). Thus, this work demonstrated that suppression of grain growth can also be obtained for commercial alumina.     

Effect of niobium doping on the densification and grain growth in alumina

The effect of niobium doping on the densification and grain growth of nano-sized α-Al₂O₃ powders during sintering has been investigated. The dopant concentration added ranged from 0.1 to 0.5 mol%. It was observed that addition of niobium oxide could improve the densification of the pure alumina with a lower sintering temperature, a shorter sintering time. The effect is strengthened by increasing the amount of dopant. It also demonstrated that niobium dopant significantly promotes the grain growth of alumina during sintering and the grain size of alumina increases with increasing the amount of dopant in the added range.     

Improved resistance of alumina to mild wear by aluminium titanate additions

The wear behaviour of an alumina (Al₂O₃)–aluminium titanate (Al₂TiO₅) composite containing 10 vol.% of second phase is studied and compared to that of single phase alumina. A careful control of the microstructure has been done in order to compare materials with similar alumina grain sizes. Wear tests have been performed on a pin on disk tribometer with an alumina ball as pin, at room temperature, under a normal force of 10 N and at sliding speeds from 0.06 to 0.15 m/s. Extensive analyses of the microstructural modifications due to wear have been done by a combination of field emission scanning microscopy and confocal microscopy.Mild wear conditions were attained for both materials. The main wear mechanism identified in both materials involves the formation of a hydroxide film and its cracking and delamination. The composite specimens presented increased wear resistance compared to the single phase ones.     

Wear properties and microstructures of alumina matrix composite ceramics used for drawing dies

The alumina matrix ceramics used for drawing dies were prepared by hot-press sintering method. The ceramics materials were made of Al₂O₃/TiC, Al₂O₃/(W,Ti)C and Al₂O₃/Ti(C,N). Mechanical and friction properties of these materials were tested and measured. The experiments for testing friction properties were carried on wear and tear machine. Mechanisms of frictions were analyzed with scanning electron microscope. Results showed that the alumina matrix composite ceramics have good physical and mechanical properties for used as drawing dies. Measured friction coefficients of alumina matrix composite ceramics showed a trend of decline and kept the value of 0.4–0.5 with the rotating speed of 550 rpm. Alumina matrix composite ceramics have smaller wear rate, while the wear rates of Al₂O₃/TiC and Al₂O₃/(W,Ti)C decrease gradually with a rising rotation speed. The wear of alumina matrix ceramics was severe at deformation zone. The primary wear behaviors of alumina matrix ceramics are scraping and furrowing. Even though the mechanisms for wear different, abrasive and adhesive wear were found to be the predominant wear mechanisms for the ceramic drawing die.     

Reaction sintering of colloidal processed mixtures of sub-micrometric alumina and nano-titania

The fabrication of composites formed by alumina grains (95 vol%) in the micrometer size range and aluminium titanate nanoparticles (5 vol%) by reaction sintering of alumina (Al₂O₃) and titania (TiO₂) is investigated. The green bodies were constituted by mixtures of sub-micrometric alumina and nano-titania obtained from freeze-drying homogeneous water based suspensions, and pressing the powders. The optimization of the colloidal processing variables was performed using the viscosity of the suspensions as control parameter. Different one step and two step sintering schedules using as maximum dwell temperatures 1300 and 1400 °C were established from dynamic sintering experiments. Specimens cooled at 5 °C/min as well as quenched specimens were prepared and characterized in terms of crystalline phases, by X-ray diffraction, and microstructure by scanning electron microscopy of fracture surfaces.Even though homogeneous final materials were obtained in all cases, full reaction was obtained only in materials treated at 1400 °C. The microstructure of the composites obtained by quenching was formed by an alumina matrix with bimodal grain size distribution and submicrometric aluminium titanate grains located inside the largest alumina grains and at triple points. However a cooling rate of 5 °C/min led to significant decomposition of aluminium titanate. This fact is attributed to the small size of the particles and the effect of the alumina surrounding matrix.     

Dry reciprocating sliding wear behaviour of alumina-silicon carbide nanocomposite fabricated by ceramic injection molding

The wear resistance of Al₂O₃ composite with 6 vol.% of SiC nanoparticles fabricated by thermoplastic forming technology and natural sintering was studied under reciprocating dry sliding conditions and compared with the results obtained in unreinforced alumina with similar grain size obtained by hot pressing. The nanocomposite wear resistance at contact loads of 20 N corresponding to initial Hertzian contact pressures of 1.8 GPa, was found to be superior to that of the alumina by a factor of 6.     

Ultrafast high-temperature sintering of dense and textured alumina

Crystallographic texture engineering in ceramics is essential to achieve direction-specific properties. Current texture engineering methods are time-consuming, energy extensive, or can lead to unnecessary diffusion of added dopants. Herein, we explore ultrafast high-temperature sintering (UHS) to prepare dense and textured alumina using templated grain growth (TGG). From a slurry containing alumina microplatelets coated with Fe₃O₄ nanoparticles dispersed in a matrix of alumina nanoparticles, green bodies with oriented microplatelets were prepared using magnetic assisted slip casting (MASC). The effects of the sintering temperature, time and heating rate on the density and microstructure of the obtained ceramics were then studied. We found that TGG occurs for a temperature range between 1640 and 1780 °C and 10 s sintering time. Sintering at 1700 °C for 10 s led to dense and textured alumina with anisotropic grains thanks to the Fe₃O₄ coating, which did not have the time to diffuse. The highest texture and relative density were obtained with a heating rate of ~5500 °C/min, leading to texture-dependent anisotropic mechanical properties. This study opens new avenues for fabricating textured ceramics in ultra-short times.