Deformation behaviour of soft-brittle polycrystalline materials determined by nanoscratching with a sharp indenter

Polycrystalline zinc selenide (p-ZnSe) is a typical soft brittle material with important optical applications. In this work, single and repeated nanoscratching tests were performed using a Berkovich indenter along the face-forward (FF) and edge-forward (EF) directions. The morphological features of the scratched grooves and the subsurface microstructural changes in the material were characterised by scanning electron microscopy, Raman spectroscopy, and electron backscatter diffraction (EBSD). Material removal in the ductile mode was obtained in the EF scratching direction; this was accompanied by the slip lines, and the radial cracks generated along grain boundaries. In contrast, brittle fractures occurred in the FF scratching direction, resulting in radial and lateral cracks which are responsible for generating the peeling of the material. The EBSD results demonstrated that the {111} planes are the primary slip plane and secondary cleavage plane, whereas the {110} planes are the primary cleavage plane and secondary slip planes. Tensile residual stress was detected in the subsurface region of the grooves scratched along the FF direction, whereas compressive residual stress was detected in the EF scratching direction. Fishbone-like patterns were observed in the scratched grooves under all conditions, while no phase transformation was detected. This study provides insights into the fundamental material removal mechanisms of soft brittle crystals in various abrasive machining processes, such as grinding, lapping, and polishing.     

New Vitrified Bond Diamond Grinding Wheel for Grinding the Cylinder of Polycrystalline Diamond Compacts

In this work, a kind of new vitrified bond based on Li2O-Al2O3-SiO2 glass ceramics was used to bond the diamond grains, which is made into grinding wheel and the cylindrical grinding process of polycrystalline diamond compacts （PDCs） by using the new vitrified bond diamond grinding wheel was discussed. Several factors which influence the properties of grinding wheel such as amount of vitrified bond and the kinds and amount of stuff in grinding wheel were also investigated. It was found that the new vitrified bond can firmly combine diamond grains, when there are only diamonds and vitrified bond in the structure of grinding wheel, the longevity of the grinding wheel is about 2.5-3 times as that of resin bond grinding wheel for processing PDCs. The grinding size precision of PDCs can be improved from 4-0.03 mm to 4-0.01 mm because of larger Young＇s modulus of vitrified bond than resin bond. The grinding time of a PDC product can be 1.75-2.0 min from 3.25-3.5 min, so this kind of grinding wheel can save much time for processing PDCs. Also, there is hardly noise when using this new vitrified bond diamond grinding wheel to process PDCs. The amount of vitrified bond in grinding wheel influences the longevity of grinding wheel. When the size of diamond grains is 90-107 μm, the optimal amount of vitrified bond in grinding wheel is 21% （wt pct）. When the amount of vitrified bond exceeds 21%, there are many pores in grinding block, which will decrease the longevity of grinding wheel. The existence of addition stuff such as Al2O3 or SiC can reduce the longevity of grinding wheel.     

An effort in planarising microwave plasma CVD grown polycrystalline diamond (PCD) coated 4 in. Si wafers

Large area CVD grown diamond coatings should have very smooth surface in many of its applications, like microwave power transmission windows etc. Combination of mechanical and chemical forces during polishing helps to achieve desired surface roughness of the polycrystalline diamond (PCD) coatings. Authors report the variation of pressure, slurry feeding rate, addition of chemicals and the time of polishing to observe the efficiency of chemo-mechanical polishing (CMP) technique in planarising CVD diamond material grown over 100 mm diameter silicon wafers. PCD were found to be polished by bringing down the as-grown surface roughness from 1.62 µm to 46 nm at some points on large areas. One coherence scanning interferometer was used to check the roughness at different stages of CMP. Raman spectroscopy was used to evaluate the polished PCD samples in terms of their quality, internal stress at different positions on the same wafer surface after CMP process. It was found that the well polished regions were of better quality than the less polished regions on the same wafer surface. But due to coating non-uniformity of the deposited PCD grown by microwave plasma CVD over large area, CMP could not produce uniform surface roughness over the entire 100 mm diameter wafer surface. We concluded that CMP could effectively but differentially polish large area PCD surfaces, and further process improvements were needed.     

Structural analysis of polycrystalline graphene systems by Raman spectroscopy

A theoretical model supported by experimental results explains the dependence of the Raman scattering signal on the evolution of structural parameters along the amorphization trajectory of polycrystalline graphene systems. Four parameters rule the scattering efficiencies, two structural and two related to the scattering dynamics. With the crystallite sizes previously defined from X-ray diffraction and microscopy experiments, the three other parameters (the average grain boundaries width, the phonon coherence length, and the electron coherence length) are extracted from the Raman data with the geometrical model proposed here. The broadly used intensity ratio between the C–C stretching (G band) and the defect-induced (D band) modes should be used to measure samples with crystallite sizes larger than the phonon coherence length, which is found equal to 32 nm. The Raman linewidth of the G band is more appropriate to characterize the crystallite sizes below the phonon coherence length, down to the average grain boundaries width, which is found to be 2.8 nm. “Ready-to-use” equations to determine the crystallite dimensions based on the Raman spectroscopy data are given.     

Synthesis of conductive semi-transparent silver films deposited by a Pneumatically-Assisted Ultrasonic Spray Pyrolysis Technique

The synthesis and characterization of nanostructured silver films deposited on corning glass by a deposition technique called Pneumatically-Assisted Ultrasonic Spray Pyrolysis are reported. Silver nitrate and triethanolamine were used as silver precursor and reducer agent, respectively. The substrate temperatures during deposition were in the range of 300–450 °C and the deposition times from 30 to 240 s. The deposited films are polycrystalline with cubic face-centered structure, and crystalline grain size less than 30 nm. Deposition rates up to 600 Å min⁻¹ were obtained at substrate temperature as low as 300 °C. The electrical, optical, and morphological properties of these films are also reported. Semi-transparent conductive silver films were obtained at 350 °C with a deposition time of 45 s.     

Transformation of ZnO polycrystalline sheets into hexagon-like mesocrystalline ZnO rods (tubes) under ultrasonic vibration

The mesoscale assembly process is sensitive to additives that can modify the interactions of the crystal nucleus and the developing crystals with solid surfaces and soluble molecules. However, the presence of additives is not a prerequisite for the mesoscale transformation process. In this study, ZnO sheet networks were synthesized on Al foils by a hydrothermal process. Scanning electron microscopy and transmission electron microscopy images confirmed that under ultrasonic vibration, monolithic polycrystalline ZnO sheets transformed into hexagon-like mesocrystalline tubes or rods. The formation mechanism was discussed.     

PCED2.0—A computer program for the simulation of polycrystalline electron diffraction pattern

A computer program for the simulation of polycrystalline electron diffraction patterns is described. PCED2.0, an upgraded version of the previous JECP/PCED, can be used as a teaching aid and research tool for phase identification, microstructure texture analysis, and phase fraction determination. In addition to kinematical theory for diffraction intensity calculation of polycrystalline samples, Blackman two-beam dynamical correction is included. March model is used for out-of-plane and in-plane texture simulation. A pseudo-Voigt function is used for the peak profile fitting of diffraction rings. User-friendly interface is improved in the handling of experimental diffraction data and the flexibility of indexing. Application of the program for the analysis of FePt thin films is given as an example.     

Surface acoustic wave characteristics of AlN thin films grown on a polycrystalline 3C-SiC buffer layer

In this study, AlN thin films were deposited on a polycrystalline (poly) 3C-SiC buffer layer for surface acoustic wave (SAW) applications using a pulsed reactive magnetron sputtering system. AFM, XRD and FT-IR were used to analyze structural properties and the morphology of the AlN/3C-SiC thin film. Suitability of the film in SAW applications was investigated by comparing the SAW characteristics of an interdigital transducer (IDT)/AlN/3C-SiC structure with the IDT/AlN/Si structure at 160 MHz in the temperature range 30-150 °C. These experimental results showed that AlN films on the poly (1 1 1) preferred 3C-SiC have dominant c-axis orientation. Furthermore, the film showed improved temperature stability for the SAW device, TCF = −18 ppm/°C. The change in resonance frequency according to temperature was nearly linear. The insertion loss decrease was about 0.033 dB/°C. However, some defects existed in the film, which caused a slight reduction in SAW velocity.     

Growth and characterization of La1 − xAxMnO3 (A = Ag and K, x = 0.33) epitaxial and polycrystalline manganite thin films derived by sol-gel dip-coating technique

Manganite La_0.67Ag_0.33MnO₃ (LAgMO) and La_0.67K_0.33MnO₃ (LKMO) films have been obtained on a single-crystal of LaAlO₃ (100) and quartz substrates using sol-gel dip-coating technique. Structural, electrical, magnetic and magnetoresistance properties of the films have been investigated. X-ray diffraction patterns of the films grown on LaAlO₃ (100) substrate showed to be highly oriented in the direction of (100) and the films grown on the quartz substrate have perovskite with rhombohedral structure. The oriented films exhibited typical transport properties, whereas the polycrystalline films showed significantly different behaviors of the temperature dependent resistivity, magnetization and magnetoresistance (MR). The epitaxial films of the LAgMO and LKMO showed different metal-insulator transition temperature (T_MI) and the paramagnetic-ferromagnetic phase transition temperature (T_C). This was mainly due to the different ionic radius size of Ag¹⁺ and K¹⁺. The variation of T_MI and T_C was due to the different annealing temperatures and grain size effect in the polycrystalline films. The polycrystalline films showed a higher MR than the corresponding epitaxial films. An extrinsic MR has been observed at low temperatures (< 100 K) for the polycrystalline films due to the spin dependent scattering of conduction electrons at grain boundaries.