Material removal mechanisms of ceramics turned by abrasive waterjet (AWJ) using a novel approach

Ceramics are the most typical difficult-to-machine materials for high-performance applications. The paper utilized AWJ to turn the alumina ceramics. A prediction model of the depth of penetration (DOP) after AWJ turning of alumina ceramic was established. The variation trends of predicted and experimental results with respect to process parameters were consistent. Within the range of this study, it may be stated that the developed model can achieve good predictions both qualitatively and quantitatively. The material removal mechanism of alumina ceramics turned by AWJ was also analyzed. The turned surface topography analysis showed that the main removal mechanism of alumina ceramics in turning was plastic shearing at a shallow nozzle tilt angle but brittle fracture at a larger nozzle tilt angle. Through the analysis of chip morphology during AWJ turning, it was proved that the removal mechanism of ceramics turned by AWJ was mixed (brittle fracture and plastic shear). Gained predicted model and material removal mechanisms will provide more details to understand and improve the AWJ turning technology.     

Ultra-precision grinding of AlON ceramics: Surface finish and mechanisms

Aluminum oxynitride (AlON) can be effectively finished by ultra-precision grinding. In this work, the ultra-precision grinding experiment was conducted on AlON to investigate surface characteristics and material removal mechanism. The ground surface has an unusual non-uniform morphology resulted from the different material removal modes. Grazing incidence X-ray diffraction (GIXRD), nanoindentation and Electron Back-Scattered Diffraction (EBSD) were carried out to study the micro-properties of AlON. The results revealed that the micro mechanical properties vary with the grain orientation on the surface. The morphologies of ground surface are consistent in the twinned grains and change with the grain orientation. By comparing the relationship of machining size and grain size, the material removal modes of individual grains should be taken into consideration during ultra-precision grinding. Based on this, a simple theoretical model was proposed to explain the material removal mechanism of AlON under ultra-precision grinding.     

Surface treatment of wire electro-discharge machined engineering ceramics by abrasive blasting

The surface integrity of electro-discharge machined engineering ceramics and their surface modification by abrasive blasting have been investigated in this study. Flexural strength was used for evaluating the effect of the two machining processes on the surface of machined specimens. The distributions of strength data were further analyzed by the Weibull statistical method to give a quantitative measure of the surface integrity. The test results show that the mean flexural strength values of wire electro-discharge machined Syalon 501 ceramics are considerably low, only about 33-52% of the original value of the material, 825 MPa. The value of the Weibull modulus, m, is very low, in the range of 4.2-8.8. Wire electro-discharge machined ceramic surfaces are of poor surface integrity and low reliability, so it is necessary to treat them before practical application. After undergoing abrasive blasting, the mean flexural strength values were found to be 20-25% higher than those of wire-EDM pre-machined specimens. The calculated Weibull modulus, m, is in the range of 14-15.5, which is much higher than that of wire-EDM pre-machined specimens, 8.1. The test results show that with most of the abrasive blasting conditions, the ceramic specimens behave fairly consistently and their properties vary narrowly from the mean value. The Weibull modulus, in, based on these small populations, increased with decreasing compressed air pressure and grain size of the abrasives. Abrasive blasting is an effective machining procedure to modify ceramic surfaces subjected to wire-EDM.     

Deep precision machining of SiC ceramics by picosecond laser ablation

Silicon carbide (SiC) ceramic is becoming widely used in multiple industrial applications, owing to its exceptional high-temperature properties. Yet it is still a challenge to machine SiC using traditional means without causing damage due to its high hardness and brittleness. In this study, a subtractive manufacturing technique based on the use of a fiber picosecond laser was employed to remove material from the reaction bonded SiC surface or create micro-patterns with the minimum damage to the surface, maximum surface quality and precision. Multiple laser processing parameters were investigated with the purpose of obtaining deep high-quality cuts with the minimum surface roughness and the minimum amount of the re-deposited material. The heat affected zone was analyzed by grazing angle X-ray diffractometry, cross-sectional scanning electron microscopy, energy dispersive and micro Raman spectroscopy techniques. The cut shape, depth, surface roughness as well as the kerf width and re-deposition height were assessed using a 3D laser scanning microscopy. The optimum values were established for the focal position, the laser power, linear speed, wobble frequency, wobble pattern, and number of passes. This study also identified the processing parameters for shallow and deep high-precision SiC cutting at a material removal rate of ～2 mm³/min. The work demonstrated that the developed laser machining process is an efficient subtractive manufacturing tool that can be integrated into the automated precision cutting systems for machining hard ceramic materials such as SiC and alumina.     

Key technologies for laser-assisted precision grinding of 3D C/C-SiC composites

Due to their exceptional and distinctive qualities, 3D C/C-SiC composites are widely utilized in producing high-end equipment and the aerospace national defense industries. However, the hard and pseudo plastic nature of the material and its anisotropies make it challenging to process. To improve the processing quality of 3D C/C-SiC composites, laser-assisted precision grinding technology is introduced in this paper, which innovatively controls the depth of the thermally induced damage layer by adjusting the laser process parameters to reduce the hard brittleness of the material, and then the surface is created by precision grinding with a grinding wheel on this basis. Experiments on laser-induced damage, laser-assisted grinding, and diamond scratching were carried out to investigate the effect of laser parameters on material damage and the effect of laser-assisted grinding processes, with an emphasis on revealing the mechanism of material removal. The results show that laser irradiation causes complex reactions such as sublimation, decomposition, and oxidation of 3D C/C-SiC composites, resulting in SiO2 and Si and recondensed SiC, causing surface/subsurface damage. A maximum reduction in normal grinding force, tangential grinding force, specific grinding energy, and surface roughness of 35.6%, 43.6%, 43.58%, and 24.22%, respectively, compared to conventional grinding processes with laser-assisted grinding. After laser irradiation, the degree of brittle fracture in the precision grinding of workpieces is significantly reduced due to the degradation of matrix and fiber damage caused by laser irradiation, which reduces the hard and pseudo plastic properties of the material. The removal mechanism shows a trend of ductile domain removal in the grinding of thermally damaged layers, which reduces the grinding force and improves the surface quality.     

A comprehensive review of electric discharge machining of advanced ceramics

Advanced ceramics are widely used in high temperature and wear related situations due to their unique physical and chemical characteristics. With the increasing demand for ceramics, the machining techniques of ceramics become a hot and tough issue because ceramics are extremely fragile and difficult to process. Traditional mechanical machining techniques like milling, turning, and drilling are subjected to large cutting forces and heat leading to extensive tool wear and poor machining performance. Electric discharge machining (EDM) has an outstanding ability of no-contact machining brittle and hardness materials with complex shapes via generating extreme high-temperature plasma channel to melt and vaporize materials. Therefore, in this paper, the research trends of latest EDM technologies for advanced ceramic materials were comprehensively reviewed. Firstly, according to the electrical conductivity of advanced ceramics, different EDM processes were introduced in details. Secondly, the existing physical models and material removal mechanisms of EDM process of ceramics were compared and analyzed. Then the machining performance indicators, such as MRR, Ra, TWR, surface topography, and micro-structures, were respectively investigated. Additionally, the new hybrid machining techniques of EDM were presented to provide some potential for efficiently machining advanced ceramics. Eventually, this paper also discussed the challenges associated with electrical discharge machining of advanced ceramic materials, and suggested some related research areas which possibly attract significant research attentions in the future.     

Effect of mixed-shaped silica sol abrasives on surface roughness and material removal rate of zirconia ceramic cover

Zirconia ceramic, as mobile phone body-materials, will become increasingly important with the coming of 5G communication technology. Surface quality and material removal rate of zirconia ceramic cover are vital factors to determine its wide application. Therefore, mixed-shaped silica sol abrasives were prepared by ion connecting-inducting method and applied to achieve a good surface quality and a high material removal rate on zirconia ceramic cover by using chemical mechanical polishing (CMP). Mixed-shaped silica sol abrasives contained spherical and beaded shapes were measured by scanning electron microscopy (SEM). Si–O–Al bonds were formed in the mixed-shaped silica sol abrasives and were proved by X-ray photoelectron spectroscopy (XPS). Results of CMP tests showed that zirconia ceramic cover obtained a low surface roughness of 1.824 nm and an efficient material removal rate of 0.33 μm/h. Compared with traditional spherical silica sol abrasives, the polishing rate of mixed-shaped silica sol abrasives increased by 242%. Additionally, solid-phase chemical reactions happened to formed ZrSiO₄, ZrAl₂Si₂O₉ in the CMP process. Moreover, friction coefficient was tested and polishing mechanism had been explored by a contact-friction model in this work.     

Crystallographic orientation effect on the polishing behavior of LiTaO3 single crystal and its correlation with strain rate sensitivity

Lithium tantalite (LiTaO₃ or LT) single crystal has been extensively applied in the fields of electro-optical and piezoelectric devices. As a typical anisotropic material, the crystallographic orientation effect on its machining responses, i.e., surface roughness and material removal rate (MRR), is not yet well understood. In the present work, we investigated the polishing responses of the three typical crystallographic orientations for LT single crystal under a series of rotation speeds. The results showed that both the rotation speed and crystalline orientation had little effect on the quality of polished surface. While for the MRR, it was almost linearly increased with increasing rotation speed for all of the three planes, among which the enhancements of MRR on Y-42° and Y-36° planes were more pronounced than that on X-112° plane. The scratch features and friction coefficients were investigated using a nanoindentation system under various velocities. The nano-hardness values were obtained under various strain rates and, hence, the strain rate sensitivities (SRS) were determined as 0.0172, 0.0455, and 0.043 for X-112°, Y-42°, and Y-36° planes, respectively. The better mechanical properties and “plastic” removal mechanism of X-112° plane resulted in the lower value of MRR. Also, the much lower SRS corresponded well with the relatively less sensitivity of MRR with rotation speed on X-112° plane. Results of this study suggested that the plastic parameter of SRS could provide as an excellent indicator to bridge the machining response and intrinsic deformation mechanism for brittle single crystal ceramics.     

A low temperature mechanochemical synthesis of nanostructured ZrC powder by a magnesiothermic reaction

Nanostructured ZrC powder was synthesized by the mechanochemical process in a high-energy ball mill by a magnesiothermic reaction. The effects of various amounts of Mg (stoichiometric and over-stoichiometric) on synthesis efficiency of the ZrC powders were investigated. The synthesized powders were studied by X-ray diffraction (XRD), differential thermal analysis (DTA), field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Zirconium carbide (ZrC) was produced after 30 h milling. Thermodynamic calculation showed the reaction to be a mechanically induced self-sustaining one (MSR). DTA analysis indicated that the temperature of exothermic reaction decreases significantly after 12 h milling.     

Raman scattering and infrared reflectivity study of orthorhombic/monoclinic LaTiNbO6 microwave dielectric ceramics by A/B-site substitution

The structure origin of microwave dielectric properties in A/B-site substituted LaTiNbO₆ ceramics with different crystal structures was explored by means of Raman scattering and infrared reflection spectra. Compared with the monoclinic (M) phase, the broadening of the Raman modes associated with B-O stretching and bending vibrations in the orthorhombic (O) phase was believed to be a result of the increased octahedral distortion. The Raman mode at 659?cm^?1 was assigned to the stretching vibration of B–O–B and O–B–O bonds in the unique interlayer chain structure of O phase. The Raman modes of the BO₆ tilting within 340–520?cm^?1 suggest that different octahedral connection ways should be one of the vital reasons why M and O phases own opposite-sign temperature coefficient of resonance frequency τ_f. Moreover, the result of infrared reflection spectrum fitted with a four-parameter semiquantum model indicates that the dielectric response of both M and O phases would mainly originate from the A-BO₆ external vibrations in the far-infrared frequency, particularly their AO₈ structure units. These results would provide useful insights into the structure-property relation in the RETi(Nb,Ta)O₆ (RE: rare earth) material system.     

Multiphonon scattering mechanisms to limit thermal conductivity in weberite RE3NbO7: A case study of (La1-xGdx)3NbO7 ceramics

The management of thermal conductivity is of significant scientific interest, particularly for thermal barrier coatings (TBCs). Multifarious strategies have been used to regulate heat transportation, but it is hard to achieve limit thermal conductivity at elevated temperatures. A systematical investigation of weberite (La_1-xGd_x)₃NbO₇ was thus performed, and multiphonon scattering mechanisms were introduced to achieve limit thermal conductivity (0.92 W m^?1 K^?1). Phonon point defect scattering process accounted for thermal conductivity reduction at low temperatures. Additionally, lattice softening strongly contributed to the reduction of high-temperature thermal conductivity, and solid and stiff chemical bonds were beneficial for inhibiting thermal radiative conductivity. A novel strategy was presented to modify thermal transportation property of weberite RE₃NbO₇ ceramics. Also, the hardness, toughness, and modulus were improved to promote engineering applications of weberite RE₃NbO₇. This study also illuminates novel paths for thermal management and mechanical properties manipulation of TBCs, thermoelectric materials, and microelectronics.     

Processing of CaLa2S4 infrared transparent ceramics: A comparative study of HP and FAST/SPS techniques

The densification of CaLa₂S₄ (CLS) powders prepared by combustion method was investigated by the use of Field-Assisted Sintering Technique (FAST) and Hot Pressing (HP). CLS powders were sintered using FAST at 1000°C at different pressures and heating rates and sintered by HP under 120 MPa from 800°C to 1100°C for 6 hours with a heating rate of 10°C/min. Comparison of both techniques was further realized by use of the same conditions of pressure, dwell time, and heating rate. Complementary techniques (XRD, SEM-EDS, density measurements, FTIR spectroscopy) were employed to correlate the sintering processes/parameters to the microstructural/compositional developments and optical transmission of the ceramics. Both sintering techniques produce ceramics with submicrometer grain size and relative density of about 99%. Nevertheless, HP is more suitable to densify CLS ceramics without fragmentation and also reach higher transmission than FAST. Transmission of 40%–45% was measured out of a possible maximum of 69% based on the Fresnel losses in the 8-14 μm window when HP is applied at 1000°C for 6 hours under 120 MPa. In both techniques, ceramics undergo reduction issues that originate from graphitic sintering atmosphere.     

Thermoresponsive surfaces by spin-coating of PNIPAM-co-PAA microgels: A combined AFM and ellipsometry study

Thermosensitive coatings are fabricated by spin-coating of microgels consisting of the cross-linked copolymer poly(N-isopropyl acrylamide-co-acrylic acid) (P(NIPAM-co-AA)) on silicon wafers. The microgels were synthesized with two different cross-linker molar ratios and the thin films were prepared at pH 2. At this pH the particles are negatively charged only due to the starter used for the polymerization. Scanning force microscopic (AFM) images indicate a dense packing of the particles and a strong flattening in the adsorbed state. This effect is stronger for microgels containing less cross-linker. Coatings consisting of these microgel particles show a reversible thermoresponsive swelling/shrinking in the region of the lower critical solution temperature (LCST) of NIPAM. For the ellipsometric study of this process a standard setup was modified in order to allow temperature dependent measurements of the optical thickness in a liquid cell. The temperature induced transition is sharper in the case of microgels with lower amount of cross-linker and smears out with increasing amount of cross-linker. No significant desorption of the particles occurs at pH 2, which was shown by AFM of the dried films before and after the ellipsometric measurements. In the dry state the average thickness of the prepared films is approximately 30 nm and a thickness of about 400 nm is reached in the swollen state.     

Synthesis of sodium ferrite by precursor and combustion methods: A comparative study

Thermal decomposition of sodium ferricarboxylate precursors, Na₃[Fe(L)₆].xH₂O (L = formate, acetate, propionate, butyrate) has been carried out in flowing air atmosphere from ambient temperature to 850 °C. Various physico-chemical techniques i.e. TG, DTG, DSC, XRD, IR, Mössbauer spectroscopy, etc. have been employed to characterize the intermediates and end products. After dehydration, the anhydrous complexes undergo decomposition to yield various intermediates i.e. sodium propionate/oxalate/carbonate and α-Fe₂O₃ in the temperature range 260–285 °C. A subsequent decomposition of sodium carbonate leads to the formation of sodium oxide in the successive stages. Finally, nanosized ferrites of the stoichiometry, NaFeO₂ have been obtained above 760 °C as a result of solid-state reaction between α-Fe₂O₃ and Na₂O. The same ferrite has also been prepared by the combustion method at comparatively lower temperature (400 °C) and in less time than that of precursor/conventional ceramic methods.     

Upgrading of diesel fuels and mixtures of hydrocarbons by means of oxygen low pressure plasmas: a comparative study

The oxidation of n-heptane, 1-octene, toluene, cis-decahydronaphthalene, mixtures of them, 4-phenyl-1-butene, 1,2,3,4-tetrahydronaphthalene, and three commercial diesel fuels, all in the liquid phase, by means of low pressure high-voltage oxygen plasmas was studied. Oxygen pressure was 0.2 mbar, applied power was 35 watts and reaction times ranged from 1 min to 23 h. Both individually and forming part of mixtures, olefins were the most reactive with ground-state atomic oxygen, O(³P). Olefinic double bonds reacted ca. 150 times faster than C-H bonds. Products were: epoxides and aldehydes for olefins; alcohols and ketones for alkanes; phenols for aromatics. Addition of 4.7-7.8% wt of oxygen was achieved for the diesels, depending on the particular composition, those with higher content of olefins being favoured, followed by those with higher content of alkanes.     

A comparative study on the corrosion resistance of AA2024-T3 substrates pre-treated with different silane solutions: Composition of the films formed

This work reports a comparative study on the corrosion resistance of AA2024-T3 pre-treated with three different silane solutions. The silanes used for the pre-treatments of the AA2024-T3 panels were: 1,2-bis(triethoxysilyl)ethane (BTSE), bis-[triethoxysilylpropyl]tetrasulfide (BTESPT) and γ-mercaptopropyltrimethoxysilane (γ-MPS). The analytical characterisation of the silane films was performed by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The corrosion performance of the pre-treated substrates was evaluated by electrochemical impedance spectroscopy (EIS). The results show that the pre-treatments based on silanes provide good corrosion protection of unpainted AA2024-T3. Painted substrates, previously pre-treated with the silane solutions also revealed improved corrosion resistance and good adhesion properties. Fatigue tests show that the silane pre-treatments do not affect the fatigue behaviour of the AA2024-T3. The work also discusses the formation of the protective silane films.     

Dissolution enhancement of flavonoids by solid dispersion in PVP and PEG matrixes: A comparative study

Polyvinylpyrrolidone (PVP) and poly(ethylene glycol) (PEG) solid dispersion systems with flavanone glycosides, naringin and hesperidin, and their aglycones, naringenin and hesperetin, were prepared, using solvent evaporation method, to enhance their dissolution rates that may affect their bioavailability. Drug release of both flavanone glycosides and their aglycones was directly affected by the physical state of solid dispersions. Powder‐XRD technique in combination with scanning and transmission electron microscopy revealed that PVP polymer formed amorphous nanodispersion systems with flavanone aglycones, while such systems could not be formed with their glycosides, which are bulkier molecules. Fourier transform infrared spectra suggest the presence of hydrogen bonds between PVP carbonyl groups and hydroxyl groups of both flavanone aglycones. These interactions prevent the crystallization of naringenin and hesperetin aglycones in PVP matrix. On the other hand, the ability of PEG carrier to form hydrogen bonds with flavanone glycosides or aglycones was limited, and as a result both flavanone glycosides and their aglycones remain in the crystalline form. For this reason, the solubility enhancement of PEG solid dispersions was lower than when PVP was used as drug carrier. At pH 6.8, the % release of naringenin and hesperetin from PVP/naringenin–hesperetin (80/20 w/w) solid dispersion was 100% while in PEG solid dispersions, it was not higher than 60–70%. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 460–471, 2006     

Fracture toughness of glasses and hydroxyapatite: A comparative study of 7 methods by using Vickers indenter

Numerous methods have been proposed to estimate the indentation fracture toughness Kic for brittle materials. These methods generally uses formulæ established from empirical correlations between critical applied force, or average crack length, and classical fracture mechanics tests. This study compares several models of fracture toughness calculation obtained by using Vickers indenters. Two optical glasses (Crown and Flint), one vitroceramic (Zerodur) and one ceramic (hydroxyapatite) are tested. Fracture toughness and hardness are obtained by using instrumented Vickers indentation at micrometer scale. Young's moduli are obtained by instrumented Berkovich indentation at nanometer scale. Fracture toughness is calculated with models involving crack length measurements, and by models free of crack length measurements by considering critical force, chipping, pop-in. Finally, method based on the cracking energy, commonly employed for coated materials is also used.The aim of this work is to compare seven methods, which enable the facture toughness determination, on four brittle materials. To do so, it was necessary to determine some specific constant in the case of Vickers tip use.On the one hand, results show that methods using crack length, critical force, edge chipping or pop-in lead to comparable results, and the advantages and drawbacks are highlighted. On the other hand, the indentation energy method leads to underestimated results of about 20%.     

A comparative study of the structure,defects, optical,dielectric, and magnetic properties of GdMnO3 multiferroic ceramics synthesized by solid-state reaction and sol-gel methods

In this work, GdMnO₃ ceramics were synthesized by solid state reaction and sol-gel methods, and the structure, defects and optical, dielectric and magnetic properties of the synthesized samples were comparatively investigated. The samples synthesized by different methods show a single phase structure without any detectable impurities. The SEM results suggest that the particle size of the specimen obtained by the solid phase route is on the micron scale, while that of the specimen fabricated by the sol-gel route is on the nanometer scale. Compared with the ceramic fabricated by solid-state reaction technology, the specimen synthesized by sol-gel technique possesses lower oxygen vacancies and Mn²⁺ concentration, and Mn³⁺ concentration. The positron annihilation analyses show that the cation vacancy concentration of the specimen synthesized by the solid phase approach is higher than that of the specimen synthesized via the sol-gel approach. The compound obtained by the solid phase reaction has better dielectric properties than that obtained with the sol-gel method. The magnetic transition temperature and the effective magnetic moment are influenced by the Mn ion valence state in GdMnO₃. The stronger magnetization of the ceramic synthesized via the sol-gel approach is associated with the lower concentration of cation vacancies.