Microstructure and tribological behavior of suspension plasma sprayed Al2O3 and Al2O3-YSZ composite coatings

Several alumina and alumina-zirconia composite coatings were manufactured by suspension plasma spraying (SPS), implementing different operating conditions in order to achieve dense and cohesive structures. Temperatures and velocities of the in flight particles were measured with a commercial diagnostic system (Accuraspray®) at the spray distance as a function of the plasma operating parameters. Temperatures around 2000 °C and velocities as high as 450 m/s were detected. Hence, coatings with high amount of α-alumina phase were produced. The microstructure evolution according to the spray parameters was studied as well as the final tribological properties showing efficient wear resistance.     

Phase formation of Al2O3/Ti(C,N)–NiTi composite

This study investigates the addition of NiTi shape memory alloy into Al₂O₃/Ti(C,N) ceramic as a toughening agent. The reported study is focused on the thermal–chemical interaction between the NiTi additive and the ceramic matrix. It is found that at the sintering temperature of 1673 K, the NiTi alloy is melted and the Ti content of the molten alloy react with the ceramic matrix to form TiC, leaving behind Ni which resolidifies into elliptical inclusions with distinctive mutlizone structures in the matrix of the ceramic. These findings provide useful guidance for NiTi–ceramic composite design.     

Ultrasonic machining of Al2O3/LaPO4 composites

Al₂O₃/LaPO₄ composites of varying compositions were drilled on an ultrasonic machine with low carbon steel tools (solid and hollow), in order to evaluate the response to machining. Vickers hardness for different compositions indicate critical load dependency on LaPO₄ content. Significance of LaPO₄ content on material hardness highlights the critical content for good sinterability. X-ray diffraction was done to study the phase content. Acoustic emission (AE) signals emitted by the work piece during machining was also analyzed. Ultrascan inspection was carried out to check for any internal defects. The data presented in the paper illustrate the significance of LaPO₄ addition on machinability of Al₂O₃/LaPO₄ composites in terms of MRR, AE response and hole geometry and associated defects.     

Wear characteristics of TiC, Ti(C,N), TiN and Al2O3 coatings in the turning of conventional and Ca-treated steels

The wear characteristics of single layers of TiC, Ti(C,N), TiN and Al₂O₃ were investigated during turning of conventional and Ca-treated quenched and tempered Al-killed steels. The experimental coatings were deposited using chemical vapour deposition (CVD) or moderate temperature CVD (MTCVD) on cemented carbide substrates of a single composition and the coatings were of similar thicknesses (7 ± 1 μm). The wear mechanisms and layer formation were studied using scanning electron microscopy, optical microscopy and X-ray diffraction. Inclusion modification appeared to be an effective means of enhancing machinability and all experimental coatings exhibited about 20% better performance as a result of Ca-treatment. In particular, the crater wear of the experimental coatings — excluding Al₂O₃ — was clearly reduced. Comparative cutting tests revealed important differences between the coating materials. Wear mechanisms of the experimental coatings are discussed in detail.     

Mechanical property and cutting performance of yttrium-reinforced Al2O3/Ti(C,N) composite ceramic tool material

Effects of yttrium on the mechanical property and the cutting performance of Al₂O₃/Ti(C,N) composite ceramic tool material have been studied in detail. Results show that the addition of yttrium of a certain amount can noticeably improve the mechanical property of Al₂O₃/Ti(C,N) ceramic material. As a result, the flexural strength and the fracture toughness amount to 1010 MPa and 6.1 MPam^1/2, respectively. Cutting experiments indicate that the developed ceramic tool material not only has better wear resistance but also has higher fracture resistance when machining hardened #45 steel. The fracture resistance of the yttrium-reinforced Al₂O₃/Ti(C,N) ceramic tool material is about 20% higher than that of the corresponding ceramic tool material without any yttrium additives.     

Effect of machining fluid on the process performance of electric discharge milling of insulating Al2O3 ceramic

Wire electric discharge machining (WEDM) and electrical discharge machining (EDM) promise to be effective and economical techniques for the production of tools and parts from conducting ceramic blanks. However, the manufacturing of insulating ceramic blanks with these processes is a long and costly process. This paper presents a new process of machining insulating ceramics using electrical discharge (ED) milling. ED milling uses a thin copper sheet fed to the tool electrode along the surface of the workpiece as the assisting electrode and uses a water-based emulsion as the machining fluid. This process is able to effectively machine a large surface area on insulating ceramics. Machining fluid is a primary factor that affects the material removal rate and surface quality of the ED milling. The effects of emulsion concentration, NaNO₃ concentration, polyvinyl alcohol concentration and flow velocity of the machining fluid on the process performance have been investigated.     

Cutting performance and failure mechanisms of an Al2O3/WC/TiC micro- nano-composite ceramic tool

An Al₂O₃-based composite ceramic tool material reinforced with WC microparticles and TiC nanoparticles was fabricated by using hot-pressing technique. The cutting performance, failure modes and mechanisms of the Al₂O₃/WC/TiC ceramic tool were investigated via continuous and intermittent turning of hardened AISI 1045 steel in comparison with those of an Al₂O₃/(W, Ti)C ceramic tool SG-4 and a cemented carbide tool YS8. Worn and fractured surfaces of the cutting tools were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The results of continuous turning revealed that tool lifetime of the Al₂O₃/WC/TiC ceramic tool was higher than that of the SG-4 and YS8 tools at all the tested cutting speeds. As for the intermittent turning, tool life of the Al₂O₃/WC/TiC ceramic tool was equivalent to that of YS8, but shorter than that of the SG-4 at lower cutting speed (110 m/min). However, tool life of the Al₂O₃/WC/TiC ceramic tool increased when the cutting speed increased to 170 m/min, becoming much longer than that of the SG-4 and YS8 tools. The longer tool life of the Al₂O₃/WC/TiC composite ceramic tool was attributed to its synergistic strengthening/toughening mechanisms induced by the WC microparticles and TiC nanoparticles.     

Study of polycrystalline Al2O3 machining cracks using discrete element method

Discrete element method (DEM) was introduced to simulate crack initiation and propagation of polycrystalline alumina during the brittle model machining process. A bonded particle model (BPM) was employed in the DEM simulations procedure to generate a particle assembly system similar to the micro-structure of the polycrystalline alumina. Particle and parallel bond properties, which were calibrated through a series of numerical tests, were subsequently used in the simulations of polycrystalline alumina cutting process and scratching tests. It is found that the cracks initiated right under or in front of the machining tool. There were many micro-cracks remained on the machined surface, some of them propagated downwards to form macro-cracks or forwards to lead material removal. Both DEM simulations and acoustic emission measure experiments have found that the fracture became acute when the normal and the tangential force changed suddenly, causing the crack number to increase. In 3D DEM scratching simulation, the surface cracks length and subsurface cracks depth linearly increased with the scratching depth, the value agreed well with the experimental results, and the surface-damage width decreased gradually with the depth to the surface, looking like half of a coin.     

Selective single pulse femtosecond laser removal of alumina (Al2O3) from a bilayered Al2O3/TiAlN/steel coating

We studied surface modification of a double layer protective coating on steel induced by single fs laser pulse irradiation in ambient air. The outer alumina (Al₂O₃) layer, which protects against aggressive environments, was 1.7 μm thick and the titanium aluminum nitride (TiAlN) layer in contact with the steel surface had a thickness of 1.9 μm. The pulses (λ = 775 nm, τ = 200 fs) were generated by a Ti:sapphire laser source. The pulse energy was varied from 0.32 μJ to 50 μJ, corresponding to an incident laser fluence of 0.11 J cm^− 2 to 16.47 J cm^− 2. The surface damage threshold was found to be 0.20 J cm^− 2 and the alumina layer removal was initiated at 0.56 J cm^− 2. This selective ablation of alumina was possible in a wide range of fluences, up to the maximum applied, without ablating the TiAlN layer beneath.     

TiN/Cr/Al2O3 and TiN/Al2O3 hybrid coatings structure features and properties resulting from combined treatment

New experimental results are presented on the structure and the elemental and phase composition of hybrid coatings, which were deposited on a substrate of AISI 321 stainless steel using a combination of plasma-detonation, vacuum-arc and subsequent High-Current Electron Beam (HCEB) treatment. We found that an increase in energy density intensified mass transfer processes and resulted in changes in aluminum oxide phase composition (γ → α and β → α). Also we observed the formation of a nanocrystalline structure in Al₂O₃ coatings. Electron beam treatment of a hybrid coating surface induced higher adhesion, decreased the intensity of surface wear and increased corrosion resistance in a sulphuric acid solution. The corrosion resistance of the coatings was studied in several electrolytic solutions (0.5 M H₂SO₄, 1 M HCl, 0.75 M NaCl) using electrochemical techniques. In most cases the corrosion resistance was improved, except those in NaCl solutions. The nano-hardness of the protecting coating was 13 GPa before electron beam melting and 9 GPa after it (as a result of TiN and Al₂O₃ sub-layers mixing).     

Oxidative recession, sulfur release, and Al2O3 spallation for Y-doped alloys

Some spallation of alumina scales occurred after long-term oxidation of Rene'N5+Y and immersion in water. It is proposed that 0.15 monolayers of sulfur segregation is enabled as the surface is consumed by oxidation for every 1 mg/cm² weight gain and a bulk sulfur content of 5 ppmw S.     

On the wide range of mechanical properties of ZTA and ATZ based dental ceramic composites by varying the Al2O3 and ZrO2 content

In this work the influence of pressureless sintering on the Vickers hardness and fracture toughness of ZrO₂ reinforced with Al₂O₃ particles (ATZ) and Al₂O₃ reinforced with ZrO₂ particles (ZTA) has been investigated. The ceramic composites were produced by means of uniaxial compacting at 50 MPa and the green compacts were heated to 1250 °C using a heating rate of 10 °C min⁻¹, then to 1500 °C at 6 °C min⁻¹ and maintained at this temperature during 2 h. After sintering, relative density over 94%, hardness values between 9.5 and 21.9 GPa, and fracture toughness as high as 3.6 MPa m^1/2 were obtained. The presence of TZ-3Y particles on the grain boundaries suggests that they inhibit notably the alumina grain growth. The grain sizes of pure Al₂O₃ and TZ-3Y as well as Al₂O₃ and TZ-3Y in the 20 wt% Al₂O₃+80 wt% TZ-3Y composite were 1.27 ± 0.51 μm, 0.57 ± 0.12 μm, 0.65 ± 0.19 μm and 0.41 ± 0.14 μm, respectively. The 20 wt% Al₂O₃ + 80 wt% ZrO₂ + 3 mol% Y₂O₃ (TZ-3Y) composite showed a hardness of 16.05 GPa and the maximum fracture toughness (7.44 MPa m^1/2) with an average grain size of 0.53 ± 0.17 μm. On the other side, the submicron grain size and residual porosity seem to be responsible for the high hardness and fracture toughness obtained. The reported values were higher than those obtained by other authors and are in concordance with international standards that could be suitable for dental applications.     

Feasibility study of rotary electrical discharge machining with ball burnishing for Al2O3/6061Al composite

This study investigates the feasibility and optimization of a rotary EDM with ball burnishing for inspecting the machinability of Al₂O₃/6061Al composite using the Taguchi method. Three ZrO₂ balls attached as additional components behind the electrode tool offer immediate burnishing following EDM. Three observed values (machining rate, surface roughness and improvement of surface roughness) are adopted to verify the optimization of the machining technique. In addition, six independent parameters are chosen as variables for evaluating the Taguchi method; these variables are categorized into two groups: (1) electrical parameters, i.e. peak current, pulse duration and non-load voltage; and (2) non-electrical parameters, i.e. flushing pressure of dielectric, rotational speed of electrode and residual height of hump. Experimental results indicated a feasible technique for applying rotary EDM with ball burnishing in machining the Al₂O₃/6061 composite. Optimization of this technique is also discussed.     

An evaluation of Al2O3-ZrO2 composites produced by coprecipitation method

The objective of this work is to produce Al₂O₃-ZrO₂ composite from nano-sized powders processed by coprecipitation method. Al₂O₃ and mixture of Al₂O₃ + 10 wt.% ZrO₂ precipitated successfully by chemical route from aluminum sulfate and zirconium sulfate were pressed under uniaxial compression of 170 MPa and sintered at 1600 °C for 1 h. SEM investigations revealed that, pure alumina sample has a microstructure with coarse grains which anisotropically grown up to 30-40 μm in size. In alumina-zirconia composite, the structure consists of very fine equiaxed grains of typically 2 μm in which zirconia precipitates were uniformly dispersed. By adding zirconia to alumina, hardness and indentation fracture toughness were increased from 11.6 GPa to 16.8 GPa and from 3.2 MPa m^1/2 to 4.9 MPa m^1/2, respectively. Improvement in fracture toughness was attributed to bridging effects of zirconia particles as well as transformation toughening.     

Mechanical properties of the plasma-sprayed Al2O3/ZrSiO4 coatings

The mechanical properties of plasma-sprayed Al₂O₃/ZrSiO₄ coatings were investigated by indentation-based techniques. Two types of feedstock were used to prepare the coatings: spray-dried powders and plasma-spheroidized powders. A 100-kW direct current (d.c.) thermal plasma system was employed. The values obtained were found to exhibit a close relationship with the microstructure of the as-sprayed coatings, which composed of zircon, alumina, amorphous silica and tetragonal zirconia. The coatings produced with plasma-spheroidized powders had higher microhardness, Young's modulus and fracture toughness than that produced with the spray-dried powders. The coatings produced with plasma-spheroidized powders by a 100-kW computerized system at 15 kW of net plasma energy had the best mechanical properties, while those deposited at 19 kW of net plasma energy had the worst properties due to the high density of cracks in the coatings.     

Precision ultrasonic machining process: a case study of stress analysis of ceramic (Al2O3)

Recent study and development activity in the field of precision ultrasonic machining (USM) process has focused on the ceramic (Al₂O₃). This paper first analyzes the USM process, mechanism, dynamics, and trends. It then discusses, in detail, the USM process applications on the ceramic (Al₂O₃).     

Microstructure and composition analysis in plasma sprayed coatings of Al2O3/ZrSiO4 mixtures

Plasma spraying of Al₂O₃/ZrSiO₄ was performed using spray dried and plasma spheroidised powder feedstock. The mixtures were sprayed using different spray stand-off distances and plasma power levels. X-Ray diffraction (XRD) was used to characterise the phase composition and scanning electron microscopy (SEM) examined the morphology of the sprayed surface and polished cross-sections. The results showed that the plasma spray process parameters played an important role in the final outcome of microstructures of the coatings. The coatings produced with spheroidised powders displayed a much denser structure than those produced with the spray-dried powders. The phase composition analysis showed the presence of amorphous phases in addition to crystalline alumina, zircon and tetragonal (t) zirconia (ZrO₂). Transmission electron microscopy (TEM) showed that amorphous phases and t-ZrO₂ crystals with particle size 100–200 nm could coexist within a single splat due to the relatively low local cooling rate.     

Thermal shock fatigue behavior of TiC/Al2O3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt.% TiC/Al₂O₃ composites were studied. The effect of TiC and Al₂O₃ starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference (ΔT) and number of thermal cycles (N) on fatigue crack growth (Δa). The mechanical properties and thermal fatigue resistance of TiC/Al₂O₃ composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al₂O₃ composites is due to a “true” cycling effect (thermal fatigue). Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.     

An investigation of laser-assisted machining of Al2O3 ceramics planing

Laser-assisted machining (LAM), an alternative method of fabricating difficult-to-machine materials, uses primarily laser power to heat the local area (without necessarily evaporating or melting any material) before the material is removed. It not only efficiently reduces the cutting force during the manufacturing process but also improves the machining characteristics and geography with regard to difficult-to-machine materials, especially structural ceramics.This study on the application of laser-assisted machining to Al₂O₃ ceramics examines the measurements of cutting force and workpiece surface temperature as well as surface integrity and tool wear. Specifically, it uses the lattice Boltzmann method (LBM) to calculate the temperature distribution inside the ceramic workpiece during the LAM process and ensure that the laser energy causes no subsurface damage. The experimental results reveal that the LAM process efficiently reduces the cutting force by 22% (feed force) and 20% (thrust force) and produces better workpiece surface quality than conventional planing.     

Preparation of nanostructured Ni/Al2O3 composite coatings in high magnetic field

Ni/Al₂O₃ composite coatings were prepared by a novel method from a modified Watt's type electrolyte containing nano-Al₂O₃ particles, where a high magnetic field was imposed in the direction parallel to an electrolytic current instead of mechanical agitation. Effects of magnetic field on the content of particles, surface morphology, microhardness and wear resistance of plating layer were investigated. It was found that the high magnetic field played an important role in the formation of composite coatings. The amounts of nano-Al₂O₃ particles in the composite coating increased with increasing of magnetic flux density and reached a maximum value at 8 T, then reduced slightly. The microhardness and wear resistance of the nanocomposite coatings also enhanced with increasing of magnetic flux density as compared to that of pure Ni coating fabricated in the absence of magnetic field. That was because the co-deposited nano-Al₂O₃ particles were uniformly distributed in the Ni matrix and contributed to greatly increase the microhardness and wear resistance of the composite coatings. Moreover, the mechanism of action of high magnetic field was discussed preliminarily.