Tribological properties of structural ceramics

The tribological and lubricated behavior of both oxide and nonoxide ceramics are reviewed in this chapter. Ceramics are examined in contact with themselves, other harder materials and metals. Elastic, plastic and fracture behavior of ceramics in solid state contact is discussed. The contact load necessary to initiate fracture in ceramics is shown to be appreciably reduced with tangential motion. Both friction and wear of ceramics are anisotropic and relate to crystal structure as has been observed with metals. Grit size effects in two and three body abrasive wear are observed for ceramics. Both free energy of oxide formation and the d valence bond character of metals are related to the friction and wear characteristics for metals in contact with ceramics. Surface contaminants affect friction and adhesive wear. For example, carbon on silicon carbide and chlorine on aluminum oxide reduce friction while oxygen on metal surfaces in contact with ceramics increases friction. Lubrication increases the critical load necessary to initiate fracture of ceramics both in indentation and with sliding or rubbing. Ceramics compositions both as coatings and in composites are described for the high temperature lubrication of both alloys and ceramics

Adsorption and release of BMP-2 on nanocrystalline apatite-coated and uncoated hydroxyapatite/b-tricalcium phosphate porous ceramics

The association of bone morphogenetic proteins (BMPs) with calcium phosphate bioceramics is known to confer them osteoinductive properties. The aim of this study was to evaluate the surface properties, especially regarding recombinant human BMP-2 (rhBMP-2) adsorption and release, of commercial sintered biphasic calcium phosphate ceramics after coating with biomimetic nanocrystalline apatite. The raw and coated ceramics exhibited similar macroporous structures but different nanometer-sized pores contents. Both types of ceramics showed Langmuir-type adsorption isotherms of rhBMP-2. The coating noticeably increased the rate of adsorption and the total amount of growth factor taken up, but the maximum coverage per surface area unit as well as the affinity constant appeared lower for coated ceramics compared with raw ceramic surfaces. The limited advantage gained by coating the ceramics can be assigned to a lower accessibility of the surface adsorption sites compared with the raw ceramics. The quantity of rhBMP-2 spontaneously released in cell culture medium during the first weeks was lower for coated samples than for uncoated ceramics and represented a minor fraction of the total adsorbed amount. In conclusion, the nanocrystalline apatite coating was found to favor the adsorption of rhBMP-2 while providing a mean to fine tune the release of the growth factor

Aggregates in Clay Bodies - A Research Project

An article outlining the origins of my current research. Featured in international ceramics magazine: Ceramics Technical. Originally published in 200

Microstructure of Ba1−xLaxTiO3−δ ceramics sintered by spark plasma sintering

Nano-sized Ba1−xLaxTiO3 (0.00 ≤ x ≤ 0.14) powders were prepared by a coprecipitation method and calcined at 850 °C in air. The corresponding ceramics were obtained by Spark Plasma Sintering (SPS) at 1050 °C. These ceramics are oxygen deficient and are marked as Ba1−xLaxTiO3−δ. Both powders and ceramics were characterized by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The effect of lanthanum concentration on the densification behavior, on the structure and the microstructure of the oxides was investigated. Average grain sizes are comprised between 54 (3) nm and 27 (2) nm for powders, and 330 (11) nm and 36 (1) nm for ceramics according to the La-doping level. Powders crystallize in the cubic (or pseudo-cubic) perovskite phase. The structure of ceramics consists in a mixture of cubic (or pseudo-cubic) and tetragonal perovskite type phases. As the lanthanum content increases, the tetragonality of the samples decreases, as well as the grain size

Ultrafast laser welding of ceramics.

Welding of ceramics is a key missing component in modern manufacturing. Current methods cannot join ceramics in proximity to temperature-sensitive materials like polymers and electronic components. We introduce an ultrafast pulsed laser welding approach that relies on focusing light on interfaces to ensure an optical interaction volume in ceramics to stimulate nonlinear absorption processes, causing localized melting rather than ablation. The key is the interplay between linear and nonlinear optical properties and laser energy-material coupling. The welded ceramic assemblies hold high vacuum and have shear strengths comparable to metal-to-ceramic diffusion bonds. Laser welding can make ceramics integral components in devices for harsh environments as well as in optoelectronic and/or electronic packages needing visible-radio frequency transparency

Improved method for producing metal-reinforced ceramics

Vacuum impregnation process produces metal-reinforced ceramics with only 3 percent void space volumes. Method may be used to produce metal-reinforced ceramics for high temperature or structural applications such as furnace supports and armor

OXIDATION BEHAVIOUR OF POLYMER-DERIVED CERAMICS

International audienceFor all chemical systems, regular (parabolic) oxidation rates are observed provided PDC are inherently stable, i.e. (i) Tp is sufficiently high to avoid the H2/H2O release from the pre-ceramic and (ii) the system is thermochemically stable to prevent decomposition, e.g., of oxycarb(onitr)ides+free C into SiO, CO, (N2). For instance, oxygen-rich Si-C-O ceramics should not be used over about 1200°C. Also, whereas a passive oxidation regime is observed for Si-C-N up to 1400°C, the severe increase of the oxidation rate at 1500°C is likely due to the reaction between silicon nitride and free carbon. The thermal stability of the Si-B-C-N system is significantly higher. The bubble formation in the oxide observed at 1500°C is more likely related to the overpressure of oxidation products at the interface, than that of decomposition gases. A harmful oxidation regime of the ceramic may also be observed if the free carbon phase is too abundant, segregated (for high Tp), if the silicon oxycarb(onitr)ide reactivity is too low and the oxide not protective (i.e., at low temperature).Within the intrinsic thermal stability domain, the parabolic rate Kp depends essentially on the nature of the oxide. All Si-C-(O) PDC display similar oxidation behaviours (Ea~100kJmol-1). Conversely, the increase of the nitrogen concentration in Si-C-N-(O) ceramics gives rise to an increase of Ea (up to~300kJmol-1) and decrease of Kp at low temperatures, close to the values for Si3N4. The influence of further heteroelements is variable. Only a slight decrease of Kp was noticed at 1350°C after the addition of ZrO2 in Si-C-N PDC, assigned to a free carbon concentration effect. Conversely for T≥1000°C, the addition of aluminium in Si-C-N PDC leads to a remarkable decrease of the oxidation rate after a transient stage, which was explained by the modification of the SiO2 network. However, this peculiar high temperature transitory regime and particularly the high initial oxidation rates, close to those for SiC, still have to be fully elucidated.The role of boron in the oxidation rate of Si-B-C-N ceramics is particularly complex. The exceptionally low oxidation rates initially reported might have been somehow underestimated for several reasons, e.g., the low oxide/ceramic volume ratio, the borosilicate viscous flow and/or the B2O3 volatilization …). Furthermore, the dual B-N-O/SiO2 layer, though observed by few authors, was not clearly demonstrated to slow down the O2-inward diffusion. More recent studies reported Kp values close to those for SiC and Si3N4 at 1500°C, concluding to a common rate limiting regime, though significantly complicated by the combination of the above-mentioned effects (e.g., bubble formation). A further addition of aluminium in the Si-B-C-N ceramic was detrimental to the oxidation resistance at 1500°C, indicating no sign of B2O3 stabilization. Clearly there is a lack of data on the oxidation behaviour of Si-(X)-B-C-N PDC at low and intermediate temperatures (800-1000°C). This is regrettable since the use of these borosilicate formers may be valuable in crack healing within this particular temperature range.Several other features besides plain oxidation should be carefully examined to appraise a new PDC composition for a high temperature structural application. In real use, this component is likely to be associated with different materials, submitted to other corrosive species than O2 and often to stress. The reactivity between the oxide products and the other nearby materials, the corrosion under a H2O environment and the delayed fracture in these aggressive media appears therefore particularly worth considering

Rare-earth ions doped transparent oxyfluoride glass-ceramics

In recent years, rare-earth ions doped transparent oxyfluoride glass-ceramics have attracted great attentions for their low phonon energy environments of fluoride nanocrystals and high chemical and mechanical stabilities of oxide glassy matrix. In this chapter, firstly, the crystallization behaviors of the transparent glass ceramics containing CaF2 nanocrystals are presented to demonstrate the controllable microstructure evolution of nano-composites. Secondly, the optical properties of the newly developed transparent glass-ceramics containing β-YF3 nanocrystals are systematically reviewed. The rare-earth ions are inclined to partition into the YF3 nanocrystals after crystallization. Through variation of the rare-earth doping and control of the microstructures, the glass-ceramics could exhibit high-stimulated emission cross-section, broadband near infrared emission, high efficient ultraviolet upconversion emission and bright white light emission, indicating their potential multifunctional applications in solid state laser, upconversion, optical amplifier, three-dimensional display, and so on