Theoretical hardness of the cubic BC2N

Vickers hardness of the cubic BC₂N has been investigated using the microscopic hardness model, in which the parameters have been obtained using first principles calculations. Ionicities of the chemical bonds in the cubic BC₂N depend on their surrounding chemical environments, which are included in the hardness calculations of the cubic BC₂N using the population ionicity scale. For the four selected configurations of the cubic BC₂N, the theoretical Vickers hardness has been found to lie between 70 and 72 GPa, consistent with the experimental value of 76 GPa. According to our calculations, it should be the cubic BC₂N that ranks second among the superhard materials instead of the cubic BN.     

Enhanced performance of HFCVD nanocrystalline diamond self-mated tribosystems by plasma pretreatments on silicon nitride substrates

Nanocrystalline diamond (NCD) coatings were grown by the hot-filament chemical vapour deposition (HFCVD) method on hydrogen plasma pretreated silicon nitride (Si₃N₄) substrates. The friction and wear behaviour of self-mated NCD films, submitted to unlubricated sliding and high applied loads (up to 90 N), was assessed using an oscillating ball-on-flat configuration in ambient atmosphere. The reciprocating tests revealed an initially high friction coefficient peak, associated to the starting surface roughness of NCD coatings (R_q = 50 nm). Subsequently, a steady-state regime with low friction coefficient values (0.01–0.04) sets in, related to a smoother (R_q = 17 nm) tribologically modified surface. A polishing wear mechanism governing the material loss was responsible for mild wear coefficients (k  10^− 7 mm³ N^− 1 m^− 1). The hydrogen etching procedure notably increased the film adhesion with respect to untreated surfaces as demonstrated by the high threshold loads (60 N; 3.5 GPa) prior to film delamination.     

Characterization of sepiolite as a support of silver catalyst in soot combustion

Turkish sepiolite–zirconium oxide mixtures were applied as a support for the silver catalyst in a soot combustion. Sepiolite–Zr–K–Ag–O catalyst was characterized by XRD, N₂ adsorption, SEM, TPR-H₂ and EGA-MS. The combustion of soot was studied with a thermobalance (TG-DTA). The modification resulted in a partial degradation of the sepiolite structure, however, the morphology was preserved. The adsorption of N₂ of the modified sepiolite is a characteristic for mesoporous materials with a wide distribution of pores. The specific surface area S_BET equals 83 m²/g and the pores volume is 0.23 cm³/g. The basic character of the surface centers of sepiolite is indicated by CO₂ desorption (TPD-MS) at 170 °C and at about 620 °C due to a surface carbonates decomposition. The thermodesorption of oxygen at 650–850 °C indicates the decomposition of AgO_x phases at the surface. The presence of AgO_x phases is also confirmed by TPR-H₂ spectrum (low temperature reduction peak at 130 and 180 °C). The high-temperature reduction at about 570 °C is probably related to Ag–O–M phases on the support.The soot combustion takes place at T₅₀ = 575 °C. Without silver (sepiolite–Zr–K–O) T₅₀ = 560 °C but sepiolite modified with silver (sepiolite–Zr–K–Ag–O) undergoes the same process at T₅₀ = 490 °C.     

Hydrothermal synthesis and characterization of dioctahedral smectites: A montmorillonites series

The aim of this study is to synthesize and finely characterize montmorillonite samples, dioctahedral smectites without tetrahedral charges (structural formulae Na_x(Al_(2 − x)Mg_x)Si₄O₁₀(OH)₂), to allow their use as reference samples in clay science. The montmorillonites synthesis under hydrothermal conditions at different pressures and with various layer charge deficit has been attempted. The temperature was fixed at 320 °C, the pressure parameter values were 20 MPa, 80 MPa, 120 MPa and 200 MPa. The Mg content varied from 0.25 to 0.60 per half unit cell. The reaction products have been characterized with multi-technique analyses (ICP-AES, EMP, CEC, XRD, FTIR, NMR and TGA).Montmorillonite phase was only produced at 120 and 200 MPa.At 20 and 80 MPa, the results suggest that a 0.33 and 0.16-tetrahedral charge deficit exist in the formed samples. Moreover, the octahedral occupancies are higher than two (2.15 and 2.07 at 20 and 80 MPa respectively). In these experimental conditions, the synthetic smectites are mixtures between montmorillonite, beidellite and saponite.At 120 MPa and for a Mg content of 0.25 or higher than 0.33, the synthetic products were also mixtures of smectites. Tetrahedral charge deficits of 0.11, 0.11 and 0.15 were found for Mg contents of 0.25, 0.50 and 0.60 respectively. The octahedral occupancy was also higher than 2.00.A montmorillonite phase with only octahedral charges and an octahedral occupancy near 2.00 was synthesized for a Mg content of 0.33 and at pressures equal to or higher than 120 MPa. This low charge reference smectite shows a very low amount of accessory minerals and an octahedral charge deficit only created by the presence of magnesium in the structure. This montmorillonite can be compared structurally to the most studied natural one: the montmorillonite SWy-2 from Wyoming.     

High‐Pressure Behavior of Mullite: An X‐Ray Diffraction Investigation

Using synchrotron X‐ray diffraction and diamond anvil cells we performed in situ high‐pressure studies of mullite‐type phases of general formula Al_4+2xSi_2?2xO_10?x and differing in the amount of oxygen vacancies: 2:1‐mullite (x = 0.4), 3:2‐mullite (x = 0.25), and sillimanite (x = 0). The structural stability of 2:1‐mullite, 3:2‐mullite, and sillimanite was investigated up to 40.8, 27.3, and 44.6 GPa, respectively, in quasi‐hydrostatic conditions, at ambient temperature. This is the first report of a static high‐pressure investigation of Al₂O₃–SiO₂ mullites. It was found that oxygen vacancies play a significant role in the compression mechanisms of the mullites by decreasing the mechanical stability of the phases with the number of vacancies. Elevated pressure leads to an irreversible amorphization above ~20 GPa for 2:1‐mullite and above 22 GPa for 3:2‐mullite. In sillimanite, only a partial amorphization is observed above 30 GPa. Based on Rietveld structural refinements of high‐pressure X‐ray diffraction patterns, the pressure‐driven evolution of unit cell parameters is presented. The experimental bulk moduli obtained are as follows: K₀ = 162(7) GPa with K₀′ = 2.2(6) for 2:1‐mullite, K₀ = 173(7) GPa with K₀′ = 2.3(2) for 3:2‐mullite, K₀ = 167(7) GPa with K₀′ = 2.1(4) for sillimanite.     

Superhard and superelastic films of polymeric C60

The C₆₀ thin film deposited on steel substrate was transformed by high pressure–high temperature treatment to a superhard and superelastic material. The films were studied by Raman spectroscopy in situ at 20 GPa after heating at 300°C and ex situ after the quenching. The hardness and elastic properties of the high-pressure phases have been characterized with nanoindentation. The hardness of the films were determined to be 0.5±0.1 GPa and 61.9±9 GPa for unmodified C₆₀ and HPHT treated films, respectively. The hardness of the pressurized film is higher than for cubic BN but lower than hardness values reported for ultrahard fullerite samples prepared from powders. An interesting observation was that the HPHT treated film showed an extreme elastic response with an elastic recovery of approximately 90%.     

Structure,mechanical, and thermal properties of Ti1‐xAlxN/CrAlN (x = 0.48, 0.58, and 0.66) multilayered coatings

Nano‐multilayered TiAlN/CrAlN coatings combining advantages of Ti‐Al‐N and Cr‐Al‐N are considered to be promising candidates for advanced machining processes. Here, the structure and thermal properties of Ti_1‐xAl_xN/CrAlN (x = 0.48, 0.58, and 0.66) multilayered coatings as well as referential Ti_1‐xAl_xN and Cr_0.32Al_0.68N monolithic coatings were investigated. Ti_1‐xAl_xN coatings show a structural transformation from cubic structure for x = 0.48 to mixed cubic and wurtzite structure for x = 0.58 and 0.66, and Cr_0.32Al_0.68N coating exhibits a single cubic structure. Through a multilayer arrangement with Cr_0.32Al_0.68N layers, the Ti_0.52Al_0.48N and Ti_0.42Al_0.58N layers can be stabilized in their metastable cubic structure, but the Ti_0.34Al_0.66N layer still tends to crystallize in the mixed cubic and wurtzite structure. The hardness of Ti_0.52Al_0.48N/CrAlN and Ti_0.42Al_0.58N/CrAlN coatings is higher than that of corresponding monolithic coatings regardless of as‐deposited and annealed states. Especially, after annealing at 800°C, the Ti_0.52Al_0.48N/CrAlN and Ti_0.42Al_0.58N/CrAlN coatings reach their peak hardness of ~34.2 and 32.8 GPa due to the spinodal decomposition of Ti_1‐xAl_xN layers. However, the oxidation resistance of Ti_1‐xAl_xN/CrAlN coatings is mainly up to the Al content of Ti_1‐xAl_xN layers, where only the Ti_0.34Al_0.66N/CrAlN coating can survive the 10 h exposure to air at 1000°C.     

High temperature properties of B6O-materials

B₆O is a potential superhard material with a hardness of 45 GPa measured on single crystals. Recently it was found that different oxides can be utilized as an effective sintering additive which allows densification under low pressures. In this work the effect of addition of Y₂O₃/Al₂O₃ on high temperature properties was investigated using impulse excitation technique (IET), hardness measurements and dilatometric measurements. The IET technique reveals the softening of the residual B₂O₃ in the materials without additives at 450 °C; in the materials with Y₂O₃/Al₂O₃ the softening is observed at only about 800 °C. This data agrees with the values found for different borate glasses.The materials showed no pronounced reduction of hardness at these temperatures. This is additional evidence, supporting previous observations that the material consists of pure grain boundaries between B₆O grains. Hardness values (HV5) of up to 17 GPa at 1000 °C were observed.     

Stepwise shock compression of C70 fullerene

Shock-induced phase transitions of C₇₀ fullerene are experimentally studied up to a pressure of 36 GPa and a temperature of 1200 K. Pressure–temperature histories of C₇₀ specimens are estimated and overlaid on the tentative phase diagram of C₇₀. The crystalline phase of fullerene C₇₀ with a hexagonal close-packed structure remained practically unchanged under stepwise shock compression up to а pressure of 8 GPa. Contrariwise the crystalline phase of fullerene C₇₀ with a rhombohedral structure fully converted to the phase with a cubic structure under similar conditions. Shock-induced transformation of the hexagonal phase into the face-centered cubic phase was observed at pressures in the range of 9–19 GPa. The amount of transformed material increases with the shock intensity. Upon further increase in the shock pressure, the destruction of C₇₀ molecules begins. In the sample recovered from 26 GPa, we observed the traces of C₇₀ with a face centered cubic structure only. The destruction of C₇₀ is accompanied by formation of graphitic carbon.     

DLC–ceramic multilayers for automotive applications

The present work explores the deposition of hard, wear resistant multilayer coatings, by magnetron sputtering onto Aluminium (Al) alloy substrates that are used in the automotive industry. Multilayer coatings have been manufactured to increase surface hardness and wear resistance for a commercial powder metallurgy Al alloys (Al 2618). The multilayer coating consisted of 25 bi-layers of Titanium Diboride (TiB₂) and diamond-like carbon (DLC). These DLC/TiB₂ coatings were fabricated, maintaining a constant composition wavelength (sum of two layers [λ] = 200 nm) for an array of ceramic fractions ranging from 75% to 95% by volume. The effect of the DLC content on the structure and performance (hardness and adhesion) of the films was investigated. The bi-layer thickness influences the failure patterns resulting from the scratch testing. This study has found hardness values of 27.8 GPa, with a critical load of 20 N and a friction coefficient of 0.47. As a result of these findings the multilayer with 10% of DLC was found to be a better compromise between high hardness (23.8 GPa) and high adhesion (critical load higher than 20 N) and with no signs of cracking during friction testing, proving to be a solution to be employed in components located in the upper valve train area of high performance vehicles.     

Improvement of quality factor of SrTiO3 dielectric ceramics with high dielectric constant using Sm2O3

The ceramics with the formula of Sr_1−xSm_2x/3TiO₃ (x = 0-0.5) were prepared by conventional ceramic process. A single phase of SrTiO₃-type with cubic perovskite structure was found companied with a decrease in crystal volume when x < 0.5, which was confirmed by X-ray diffraction results and Rietveld refine. At the level of x = 0.5, the sample consisted of two phases of SrTiO₃-type and Sm₂Ti₂O₇. Raman spectra were used to confirm the change of vibration to explain the variation in microwave dielectric properties. The dielectric constant decreased, the quality factor increased, and the temperature coefficient shifted negatively, when the x value increased from 0 to 0.4. When the x value located between 0.3 and 0.4, the ceramics exhibited high dielectric constant of 140-152, high quality factor (>6000).     

Influence of lattice defects on the high pressure properties of Ni0.66Mn2.34O4 NTC ceramics

The properties and hence the applications of negative temperature coefficient (NTC) oxides, Ni_xMn_3-xO₄, strongly depend on the Ni:Mn ratio, valence states, defects and cation distribution in the spinel structure. Here, the accuracy of determination of nickel and manganese cations distribution in the spinel structure of Ni_0.66Mn_2.34O₄ oxide, has been improved by taking into account the presence of tetrahedral vacancies. Additionally, the structural stability of cubic Ni_0.66Mn_2.34O₄ was investigated as a function of pressure up to 9.5 GPa using synchrotron radiation angle-dispersive X-ray powder diffraction and a diamond anvil cell. The bulk modulus and its first derivative were determined by fitting the Birch-Murnaghan equation of state (EoS) model to the experimental pressure-volume data. Rietveld refinement of the X-ray powder diffraction data reveals that the cubic spinel structure is stable upon compression to 9.5 GPa. The XRD data yielded a bulk modulus of K₀ = 126(7) GPa, with a pressure derivative K′ = 12(2). The obtained data were discussed in terms of defects in the cationic sublattice and compared with the elastic parameters of NiMn₂O₄.     

Physical Properties of La1−xEuxPO4,0 ≤ x ≤ 1, Monazite‐Type Ceramics

Synthetic La_1?xEu_xPO₄ monazite‐type ceramics with 0 ≤ x ≤ 1 have been characterized by ultrasound techniques, dilatometry, and micro‐calorimetry. The coefficients of thermal expansion and the elastic properties are, to a good approximation, linearly dependent on the europium concentration. Elastic stiffness coefficients range from 182(1) to 202(1) GPa for c₁₁ and from 53.8(7) to 61.1(4) GPa for c₄₄. They are strongly dependent on the density of the sample. The coefficient of thermal expansion at 673 K is 8.4(3)  × 10^?6 K^?1 for LaPO₄ and 9.9(3)  × 10^?6 K^?1 for EuPO₄, respectively. The heat capacities at ambient temperature are between 101.6(8) J·(mol·K)^?1 for LaPO₄ and 110.1(8) J·(mol·K)^?1 for EuPO₄. The difference between the heat capacity of LaPO₄ and the Eu‐containing solid solutions is dominated by electronic transitions of the 4f‐electrons at temperatures above 75 K.     

In situ Raman spectroscopy of pressure‐induced phase transformations in polycrystalline TbPO4, DyPO4, and GdxDy(1−x)PO4

Xenotime DyPO₄ and Gd_xDy_(1?x)PO₄ (x = 0.4, 0.5, 0.6) (tetragonal I4₁amd zircon structure) have been studied at ambient temperature under high pressures inside a diamond anvil cell with in situ Raman spectroscopy. The typical Raman‐active modes of the xenotime structure were observed at low pressures and the appearance of new Raman peaks at higher pressures indicated a phase transformation to a lower symmetry structure—likely monoclinic. Raman mode softening was observed, resulting in a line crossing at approximately 7‐8 GPa for each material and preceding the phase transformation. The onset of phase transformation for DyPO₄ occurred at a pressure of 15.3 GPa. DyPO₄ underwent a reversible phase transformation and returned to the xenotime phase after decompression. The transformation pressures of the solid solutions (Gd_xDy_(1?x)PO₄) were in the range 10‐12 GPa. The Gd_xDy_(1?x)PO₄ solid solutions yielded partially reversible phase transformations, retaining some of the high‐pressure phase spectrum while reforming xenotime peaks during decompression. The substitution of Gd into DyPO₄ decreased the transformation pressure relative to pure DyPO₄. The ability to modify the phase transformation pressures of xenotime rare‐earth orthophosphates by chemical variations of solid solutions may provide additional methods to improve the performance of ceramic matrix composites.     

A Complete Solid Solution with Rutile‐Type Structure in SiO2–GeO2 System at 12 GPa and 1600°C

High pressure and temperature synthesis of compositions made of (Si_1?x,Ge_x)O₂ where x is equal to 0, 0.1, 0.2, 0.5, 0.7, and 1 was performed at 7–12 GPa and 1200–1600°C using a Kawai‐type high‐pressure apparatus. At 12 GPa and 1600°C, all the run products were composed of a single phase with a rutile structure. The lattice constants increase linearly with the germanium content (x), which indicates that the rutile‐type phases in the SiO₂–GeO₂ system form a complete series of solid solutions at these pressure and temperature conditions. Our experimental results show that thermodynamic equilibrium state was achieved in this system at 12 GPa and 1600°C, but not at 1200°C. At lower pressures (7 and 9 GPa) and 1600°C, we observed the decomposition of (Si_0.5,Ge_0.5)O₂ into SiO₂‐rich coesite and GeO₂‐rich rutile phases. The silicon content in the rutile structure increases sharply with pressure in the vicinity of the coesite–stishovite phase transition pressure in SiO₂.     

A first-principle study on the structure, stability and hardness of cubic BC2N

The hardness of cubic BC₂N (c-BC₂N) has sparked considerable debate in the literature. First-principle calculations, guided by bond counting rules, were engaged to investigate the correlation between the stability of an alloy configuration and its hardness by searching through alloy configurations in all distinct unit cells of size up to 12 atoms. The existence of many low-energy and high-density alloy configurations with formation energies in the order of 100–200 meV/atom suggests that a mixture of high-density alloy configurations in the experimental samples is likely to occur due to the high temperature and non-equilibrium conditions offered by modern growth techniques. Theoretical analysis on elastic stiffness and Vickers hardness confirm that these stable high-density c-BC₂N alloy configurations are indeed harder than c-BN.     

The crystalline-to-amorphous transition in shock-loaded mullite Al2(Al2+2xSi2−2x)O10−x in the light of shear modulus anisotropy

We present experimental evidence for shock-wave induced amorphization in polycrystalline and single crystal mullite, Al₂^VI(Al_2+2x Si_2−2x)^IVO_10−x, at peak pressures above 35 GPa. The transition proceeds along with a network of very thin glass lamellae (planar deformation features (PDFs)) of mullite-normative composition extending parallel to low-index crystallographic planes including {1 2 0}, {2 3 0} and {1 1 0}. Cumulative microstructural evidence from the PDFs derived via analytical transmission electron microscopy suggests a shear-induced formation mechanism. Experimental PDFs match the relative minima of the calculated representation surfaces of the shear modulus suggesting that suitable PDF orientations can be derived from the elastic anisotropy of mullite. PDFs in mullite are in good agreement with those reported for naturally shocked sillimanite.Unlike the formation of shear-induced PDF-type glass lamellae in shocked mullite, the thermal decomposition of mullite following high post-shock temperatures results in a fine-grained phase assemblage consisting of corundum plus amorphous silica, and represents the most abundant transformation mechanism in the shock regime investigated (20–40 GPa). No stishovite was observed. At shock levels beyond 35 GPa thermal decomposition of mullite may occur along with PDFs within the same specimen.     

A Simple Method for Obtaining Kinetic Equations to Describe the Enzymatic Hydrolysis of Biopolymers

A procedure to obtain the overall rate of hydrolysis of biopolymers is proposed, based on the fitting of the experimental data x=f(t) to cubic spline functions and from these, by differentiation, to obtain dx/dt. The values of these dx/dt slopes are an exclusive function of the conversion, x, when E₀, S₀, pH and temperature are constant. The fitting of dx/dt versus x leads to equations of the type dxdt=a · x exp(-b · x) for the glucoamylase–starch system, where b=8·75 and a=f(E₀, T).     

High P-T phase transformations and metastability in the Zr0.5Hf0.5O2 solid-solution ceramic

High pressure-temperature (P-T) phases of the Zr_xHf_1−xO₂ (x = 0.5) solid-solution have been stabilised in a CO₂ laser heated diamond anvil cell. At room-temperature the monoclinic to orthorhombic-I structural transformation is initiated at 5-8 GPa. The X-ray diffraction (XRD) studies show these two phases coexist to above ∼15 GPa. A progressive increase in the orthorhombic-I phase abundance occurs, to culminate in full conversion at ∼20 GPa. At this lower threshold of ∼20 GPa transformation to the orthorhombic-II (cotunnite) structure can be initiated by heating in the range of 600-1200 °C. Substantial conversion to the cotunnite phase occurs in the same temperature range at 25-30 GPa. Raman signatures have been assigned to the two orthorhombic high-pressure phases, aided by the qualitative assessment of the complementary XRD data. Decompression experiments show that phase mixture composites of these high pressure structures, possibly with enhanced tribological properties, can be recovered to ambient conditions.     

Phase relations in silicon and germanium nitrides up to 98 GPa and 2400°C

Phase relations in silicon and germanium nitrides (Si₃N₄ and Ge₃N₄) were investigated using a Kawai-type multianvil apparatus and a laser-heated diamond anvil cell combined with a synchrotron radiation. The pressure-induced phase transition from the β to γ (cubic spinel-type structure) phase was observed in both compositions. We observed the coexistence of the β and γ phases in Si₃N₄ at 12.4 GPa and 1800°C, while the appearance of single phase γ-Ge₃N₄ was observed at pressures above 10 GPa. Our observations under higher pressures revealed that γ-Si₃N₄ and γ-Ge₃N₄ have wide stability fields and no postspinel transition was observed up to 98 GPa and 2400°C in both compositions. Using the room-temperature compression curves of these materials, the bulk moduli (K₀) and their pressure derivatives (K′₀) were determined: K₀ = 317 (16) GPa and K′₀ = 6.0 (8) for γ-Si₃N₄; K₀ = 254 (13) GPa and K′₀ = 6.0 (7) for γ-Ge₃N₄.