Surface texturing inside ceramic macro/micro channels

Surface textures (roughness) inside of macro/micro channels have a number of potential uses in advanced ceramic applications, including areas such as bioceramic, cooling, microreactors or friction control. The presented microtemplating method is dealing with templating of structured fibres, which results in formation of variation in surface textures and roughness. Effect of the microtemplate skin texture and ceramic slurry behaviour has been investigated and discussed. Porosity and defects of the microtemplate skin play a key role for roughness/texture of sintered microchannels if their size is larger than the ceramic grain size. The surface roughness inside the micro/macro channel can be easily changed from R_a = 0.8 μm to R_a = 4.9 μm just by changing the microtemplate skin porosity. The results demonstrate possibilities to use this method to tailor surface texture inside microchannels in a one step procedure.     

Study of light scattering on a soda lime glass eroded by sandblasting

In Saharan areas of Algeria, sandstorms can damage vehicles windshields inducing incidental light diffusion that affects the driver's visibility. Vehicles technical controllers find some difficulties with damaged windshields. The control being made visually with the naked eye, it is therefore difficult to judge when a damaged windshield is no more valid to use. In this context, we studied the influence of the surface state of a soda lime glass on the scattering of a white light. The varying parameters considered are the projected sand mass, the opening of the light beam and the distance sample-receptor. By increasing the projected sand mass up to 200 g, the optical transmission falls from 91.6 to 13% and the roughness increases from 0.035 up to 2.27 μm and then tends toward a constant level. For the as-received state, the image obtained using a CCD camera presents a net boundary and the transmission profile shows a saturation plateau. By damaging the surface, the image boundary deforms and becomes diffuse. For the highly damaged states, the image become completely blurred and the transmission profile disappears. The variation of the transmission according to roughness shows an inflection point at T = 73% and R_a = 1.5 μm. This point seems to separate two domains: a transparent field (R_a < 1.5 μm) and a blur field (R_a > 1.5 μm). The visibility limit obtained in our tests conditions is estimated at about 73%.     

New laser machining technology of Al2O3 ceramic with complex shape

A new method was developed for the fabrication of complex-shaped Al₂O₃ ceramic parts by combining laser machining and gelcasting technique. The unwanted ceramic powders parts were selectively removed by laser machining specified by a computer program, and the gelcast Al₂O₃ green bodies were machined to a designed shape by a CO₂ laser at a wavelength of 10.6 μm. The influences of solid loading, laser output power, scanning speed and nitrogen purge on the machining of green Al₂O₃ ceramic bodies were studied. The experimental parameters were optimized, the green Al₂O₃ bodies with solid loading of 40 vol% or below were easier to be machined, while the green bodies with solid loading of 45 vol% or above were hard to be further machined due to the surface sintering. Better machining quality and deeper machining depth could be obtained when the laser power is 30 W. The green Al₂O₃ bodies with complex shape were obtained by the laser machining.     

Precise micromachining of yttria-tetragonal zirconia polycrystal ceramic using 532 nm nanosecond laser

High quality micro-sized steps and blind hole structures without microcracks, chips or spatter deposition were machined on yttria-tetragonal zirconia polycrystal (Y-TZP, 3 mol% yttria) by nanosecond laser (wavelength=532 nm, pulse width ~6 ns). The diameter of blind hole is 500 μm and each step is 500±10 μm wide and 100±5 μm deep. The 1.35 mm³/min removal rate and the smooth machined surface with Ra=2.824 μm roughness depicting the high precise and efficient processing were achieved. The ablation characteristics of nanosecond laser process of Y-TZP ceramic were also studied. Based on the study, a reasonable design of the processing path for micromachining of a finer embedded step with 24±2 μm width (smaller than the 60 μm focused spot size) around the inner-wall of a 2×2 mm² cavity was developed. These results and discussion offer new possibilities in the manufacturing of bio-ceramic products by nanosecond laser with high processing quality and efficient.     

Influence of steel substrate roughness on morphology and mesostructure of TiO2 porous layers produced by template-assisted dip coating

Mesoporous titania films were prepared by template-assisted dip coating on 1.4301 stainless-steel substrates processed by grinding and spark erosion to different degrees of roughness. The influence of substrate roughness on the morphology and mesostructure of deposited films was studied. Textures produced by grinding with roughness R_a ranging from 0.10 to 0.78 μm did not noticeably affect the pore structure as confirmed by similar pore size and a single cubic mesophase formed on grinded steel. Grinding had a modest effect on the film integrity which manifested in fractures developed in the texture depressions. Greater roughness of the steel produced by spark erosion affected the micelle self-assembly process yielding two different mesophases on a substrate of 1.08 μm roughness, and resulting in a predominant loss of templated mesostructure on a rougher (R_a = 2.69 μm) substrate surface. Film surface area expressed as m² BET per m² of the substrate planar dimensions increased with substrate roughness. Higher roughness resulted in higher photocatalytic activity of crystalline films when tested in methylene blue decomposition. Given that a moderate surface texture had a negligible effect on the film mesostructure, introducing controlled substrate roughness may serve as a technique to enhance the total film surface area.     

In-Situ Determination of bridging stresses in Al2O3/Al2O3-platelet composites by fluorescence spectroscopy

Bridging stresses arising from interlocking and frictional effects in the crack wake have been quantitatively evaluated in an Al₂O₃/Al₂O₃-platelet ceramic, using in-situ microprobe fluorescence spectroscopy. Crack opening displacement (COD) profile has also been quantitatively measured in the scanning electron microscope (SEM), in order to substantiate the reliability of the piezo-spectroscopic measurements of microscopic bridging stresses. Mapping the crack wake (at critical condition for crack propagation) with a laser probe of 2 μm spatial resolution led to determine a discrete map of closure stresses over a crack extension of about 800 μm. Relatively high bridging stress values ≈350 MPa were revealed due to platelet interlocking in a near-tip bridging zone <100 μm, whereas frictional sites of lower stress magnitude <100 MPa were monitored in the crack profile farther away from the crack tip. The availability of microscopic fracture parameters like as the bridging stress distribution and the near-tip COD profile enables to quantitatively explain the rising R-curve behavior of the Al₂O₃/Al₂O₃-platelet material. Bridging stress distribution, COD profile and R-curve data are discussed in comparison with those collected in previous studies on equiaxed Al₂O₃ and toughened Si₃N₄. The present study supports the notion that crack bridging is by far the most important toughening mechanism in non-transforming ceramics.     

Fast removal of surface damage layer from single crystal diamond by using chemical etching in molten KCl + KOH solution

To fast remove the surface damage layer from single-crystal-diamond, we have developed a chemical etching process using molten KCl and KOH solution at high temperature around 1100 °C. High removal rate about R_{001} = 2.0 μm/h, R_{101} = 20 μm/h and R_{111} = 26 μm/h was achieved for the {001} sample surfaces, {101} and {111} sample edges, respectively. Laser microscope observation has confirmed that the {001} surface flatness has been greatly improved after etching and surface roughness formed during previous lift-off process has been effectively removed.     

Improving cutting quality by analysis of microstructure characteristics and solidification behaviour of recast layer formation on laser cut ceramic

We studied the microstructure characteristics and solidification behaviour of the recast layer on a 1 mm Al₂O₃ electronic ceramic substrate processed by CO₂ laser cutting to improve the cutting quality. SEM (scanning electron microscope) and CLSM (confocal laser scanning microscope) observations showed that the upper and lower regions of the recast layer consist of equiaxed grains, while columnar grains dominate in the middle region, downgrading cutting quality. We used finite element modelling (FEM) to understand the solidification mechanism and explain profile and microstructure variation along the kerf. Using this analysis, we performed experiments to study the influences of assist gas pressure and laser cutting speed on the grain size and recast layer thickness. A thin, dense recast layer of uniform thickness and microstructure resulted from the following optimal parameters: 8 bar N₂ assist gas pressure, cutting speed of 1500 mm/min, and laser fluence of 200 W.     

UV laser microprocessing and post chemical etching on ultrathin Al2O3 ceramic substrate

In this study, an Nd:YVO₄ UV laser was used for microprocessing ultrathin (125 μm) ceramic plates for use as a multi-layer microchip substrate. The effects of the UV laser microprocessing parameters, including laser power density, frequency, laser scanning speed, and pass delay on microprocessing accuracy and quality (kerf width and arithmetic average roughness R_a on the kerf sidewall) were investigated by means of a 4 × 4 orthogonal design. The key processing parameters were determined and optimized for small kerf width and minimal R_a of the kerf sidewall while retaining high production efficiency. Subsequent chemical etching of the laser processed areas was performed to reduce the kerf surface and kerf sidewall roughness by removing debris and the thin recast layer for the required size precision and post gilding treatment. The results showed that a clean surface and crack-free kerf sidewall with roughness R_a of 0.16 μm could be achieved by laser microprocessing and chemical etching.     

Microstructures and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic by laser floating zone melting with high temperature gradient

Directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic rods with high densities and low solidification defects are prepared by laser floating zone melting at solidification rate from 2 to 200 μm/s. The microstructure evolution, eutectic growth behavior and mechanical properties are investigated. At low solidification rates (<30 μm/s), the eutectic rods present a homogeneous irregular eutectic microstructure, whereas cellular microstructure containing regular lamella/rod structure is developed at higher solidification rates. The relationship is established between the eutectic interphase spacing and solidification rate, which follows the Magnin-Kurz eutectic model. The Vickers hardness (15.9–17.3 GPa) increases slightly with decreasing interphase spacing, but the fracture toughness (4.08 MPa m^1/2) shows little dependence with the solidification rate. Different crack propagation mechanisms are revealed among the indentation cracks. The flexural strength at ambient temperature reaches up to 1.14 GPa for the eutectic grown at 100 μm/s. The fracture surface analysis indicates that the surface defects are the main crack source.     

Fabrication of micro-scale textured grooves on green ZrO2 ceramics by pulsed laser ablation

The green ZrO₂ ceramics were fabricated by cold isostatic pressing. Pulsed laser ablation with a wavelength of 1064 nm was performed to fabricate micro-scale textured grooves on the surface of green ZrO₂ ceramics. The influence of laser parameters on surface quality was studied. The heat-affected zone around the machined grooves and micromorphology of laser-irradiated surface were investigated. Results showed that micro-scale textured grooves with a width of 30–50 µm and a depth of 15–50 µm on the green ZrO₂ ceramic surfaces were successfully fabricated by pulsed laser ablation. The laser parameters had a profound influence on the surface quality of micro-scale textured grooves. Better surface quality could be obtained with frequency below 40 Hz, power below 6 W, and scanning velocity above 200 mm/s. A sintering layer was found on the laser-irradiated surfaces when frequency was above 60 Hz, power was above 10 W, and scanning velocity was below 150 mm/s. Analysis of this sintering layer revealed clear melting and resolidification of ZrO₂ particles.     

Application of Box–Behnken design and response surface methodology for modeling of some Turkish coals

The aim of our research was to apply Box–Behnken experimental design and response surface methodology for modeling of some Turkish coals. As a base for this study, standard Bond grindability tests were initially done and Bond work indexes (W_i) values were calculated for three Turkish coals. The Box–Behnken experimental design was used to provide data for modeling and the variables of model were Bond work index, grinding time and ball diameter of mill. Coal grinding tests were performed changing these three variables for three size fractions of coals (−3350 + 1700 μm, −1700 + 710 μm and −710 μm).Using these sets of experimental data obtained by mathematical software package (MATLAB 7.1), mathematical models were then developed to show the effect of each parameter and their interactions on product 80% passing size (d₈₀). Predicted values of d₈₀ obtained using model equations were in good agreement with the experimental values of d₈₀ (R² value of 0.96 for −3350 + 1700 μm, R² value of 0.98 for −1700 + 710 μm and R² value of 0.94 for −710 μm). This study proved that Box–Behnken design and response surface methodology could efficiently be applied for modeling of grinding of some Turkish coals.     

High-rate ultrasonic polishing of polycrystalline diamond films

We report on fast polishing of polycrystalline CVD diamond films by ultrasonic machining in a slurry with diamond particles. The material removal mechanism is based on diamond micro-chipping by the bombarding diamond particles subjected to action of an ultrasonic radiator. The treated samples were characterized with optical profilometry, SEM, AFM and micro-Raman spectroscopy. The developed method demonstrates the polishing rate higher than those known for mechanical or thermo-mechanical polishing, particularly, the surface roughness of 0.5 mm thick film can be reduced in a static regime from initial value Ra ≈ 5 μm to Ra ≈ 0.5 μm for the processing time as short as 5 min. No appearance of amorphous carbon on the lapped surface was revealed, however, formation of defects in a sub-surface layer of a few microns thickness was deduced using Raman spectroscopy. The polishing of a moving workpiece confirmed the possibility to treat large-area diamond films.     

Microstructure and mechanical properties of Al2O3/Y3Al5O12/ZrO2 hypereutectic directionally solidified ceramic prepared by laser floating zone

Al₂O₃/Y₃Al₅O₁₂/ZrO₂ directionally solidified ceramic has been considered as a promising candidate for ultrahigh temperature structural materials due to its excellent performance even close to its melting point. In this work, laser floating zone (LFZ) solidification experiments were performed on Al₂O₃/Y₃Al₅O₁₂/ZrO₂ hypereutectic with the solidification rates between 2 μm/s and 30 μm/s. The full eutectic lamellar microstructure is obtained with hypereutectic composition. The solid/liquid interface morphology is investigated. The microstructure characteristic is discussed based on the solid/liquid interface. The variation of lamellar spacing with different compositions and solidification rates was reported and discussed by considering an irregular eutectic growth model. The maximum hardness and fracture toughness are 19.06 GPa and 3.8 MPa m^1/2, respectively. The toughening mechanism of ZrO₂ is discussed based on the scenario of the crack propagation pattern.     

Casting and characterization of LiMgPO4 glass free LTCC tape for microwave applications

The LiMgPO₄ ceramic has been prepared through the solid state ceramic route. The powder has an average particle size of 1.1 μm and BET surface area of 2.7 m² g^?1. Good dispersion of LiMgPO₄ has been achieved in ethanol/xylene mixture with the addition of 2 wt.% fish oil. The tape casting slurry of LiMgPO₄ with typical pseudoplastic behavior has been prepared and cast into thin tapes of thickness 70 μm. LiMgPO₄ green tape shows a tensile strength of 0.22 MPa and average surface roughness of 0.25 μm. The green tape has an ?_r of 3.2 and tan δ of 0.0688 at 5 GHz. The thermo-laminated tape (4 layers) sintered at 950 °C/2 h shows good microwave dielectric properties: ?_r = 6.4 and tan δ = 0.0002. LiMgPO₄ has a coefficient of thermal expansion of 10.5 ppm/°C and thermal conductivity of 7.1 W m^?1 K^?1.     

Growth of dense Ti3SiC2 MAX phase films elaborated at room temperature by aerosol deposition method

For the very first time, dense and thick films of Ti₃SiC₂, a popular MAX-phase material, were elaborated on glass substrates by the aerosol deposition method (ADM) at RT. The influence of some processing parameters on the deposition rate and morphology of the films was studied. The films revealed an adhesive interface with the substrate and a dense internal microstructure with nanocrystallites resulting from a high fragmentation of the initial powder at the impact. The film surface showed different types of structuration, from a flat to a rough one with the presence of craters, whose deepness and diameter were linked to the film thickness. The deposition rate and film morphology were both influenced by the distance of projection and the carrier gas flow. Films with thicknesses ranging from 0.1 to 16 μm were thus obtained with a high deposition rate reaching 4 μm min⁻¹, with a roughness, R_a, lower than 300 nm.     

Characterisation of the microstructure and thermal properties of Nd2Zr2O7 and Nd2Zr2O7/YSZ thermal barrier coatings

The microstructure of following thermal barrier coatings (TBC) was characterised in this paper: monolayer coatings Nd₂Zr₂O₇ and 8YSZ; a double ceramic layered (DCL) coating. Coatings were characterised by thicknesses that did not exceed 300 μm and porosities of approx. 5%. The chemical and phase composition analysis of the DCL layers revealed an external Nd₂Zr₂O₇ ceramic layer approx. 80 μm thick, a transitional zone approx. 120 μm thick and an internal 8YSZ layer 100 μm thick. For the case of the monolayer coating, the Nd₂Zr₂O₇ pyrochlore phase was the only one-phase component. The surface topography of both TBC systems was typical for plasma sprayed coatings, and compressive stress state had a value of approx. 5–10 MPa. Measurements of the thermal parameters, i.e., thermal diffusivity, point to considerably better insulative properties for both new types of layers when compared to the standard 8YSZ layers.     

Effect of XeF laser treatment on structure of nanocrystalline diamond films

Nanocrystalline diamond (NCD) films were deposited on Si substrates by microwave plasma-enhanced chemical vapor deposition (MPECVD) using methane/hydrogen/oxygen (30/169/0.2 sccm) as process gases. Subsequently a thin (0.33 μm) and a thick (1.01 μm) NCD films were irradiated with XeF excimer laser (λ = 351 nm) with 300 and 600 mJ cm^? 2 of energy densities in air. The NCD films became rougher after laser irradiations. Fraction of graphitic clusters decreased but oxygen content increased in the thin NCD film after laser irradiation. Opposite phenomena were observed for the thick NCD films. Effect of laser irradiation to oxygenation and graphitization of NCD films was correlated with structural properties of free surface and grain boundaries of the thin and thick NCD films.     

The influence of the surface texture of hydrogen-free tetrahedral amorphous carbon films on their wear performance

Hydrogen-free and predominantly tetrahedrally bonded amorphous carbon thin films (ta-C) are excellent coatings to protect surfaces from wear due to their low coefficient of friction and high hardness. Since these coatings may be several times harder than common engineering materials counterpart wear can be significant. Therefore the surface texture of the ta-C coating is critical to wear applications. While the surface roughness is an important factor, the paper shows that other surface texture parameters have to be considered as well to predict the wear performance of the coating. Wear data are compared of as deposited, polished and brushed ta-C coatings. The results show that typically referenced average values for the surface roughness such as R_a and R_z may prove insufficient to reliably predict the wear behavior of the coating. Additional parameters describing the surface texture such as the “Skewness” (R_sk) and “Kurtosis” (R_ku) can provide relevant information. For example, a brushed ta-C surface with an average roughness of R_a = 31 nm showed a tenfold improved wear performance over a polished ta-C surface with an average roughness of R_a = 10 nm. This phenomenon is explained by analyzing the R_sk and R_ku data, which prove to more closely capture the post-treatment specific changes to the surface texture of the coatings.     

Microstructure and high temperature 4-point bending creep of sol–gel derived mullite ceramics

Four-point bending creep behavior of mullite ceramics with monomodal and bimodal distribution of grain sizes was studied in the temperature range of 1320–1400 °C under the stresses between 40 and 160 MPa. Mullite ceramic with bimodal grain size distribution was prepared using aluminum nitrate nonahydrate as alumina precursor. When γ-Al₂O₃ or boehmite were used as alumina precursors, mullite grains are equiaxial with mean particle size of 0.6 μm for the former and 1.3 μm for the latter alumina precursor. The highest creep rate exhibited the sample with monomodal morphology and grains in size of 0.6 μm, which is about one order of magnitude greater than that for the monomodal morphology but with grains in size of 1.3 μm. The highest activation energy for creep (Q = 742 ± 33 kJ/mol) exhibits mullite with equiaxial grains of 1.3 μm, whereas for sample with smaller equiaxial grains the activation energy is much smaller and similar to mullite ceramics with bimodal grain morphology. Intergranular fracture is predominant near the tension surface, while transgranular more planar fracture is predominant near the compression surface zone.