In order to evaluate the effect of thermal treatments on the surface reactivity and carcinogenic potential of diatomaceous earth (DE) products, the physicochemical features of some specimens--derived by heating the same original material--were compared with their cytotoxic and transforming potency. The samples were an untreated DE (amorphous) progressively heated in the laboratory at 900 degrees C (DE 900) and 1200 degrees C (DE 1200) and a commercial product manufactured from the same DE (Chd) from which the finer fraction (< 10-microm diameter) was separated (Chd-F). Quartz (Min-U-Sil 5) and a vitreous silica (amorphous) smoothed up with hydrofluoric acid and were used as positive and negative controls, respectively. All samples were analyzed for their degree of crystallization, for their ability to release free radicals and reactive oxygen species, and for their cytotoxic and transforming potencies in Syrian hamster embryo (SHE) cells. X-ray diffractometry showed that DE 900, like DE, was still amorphous, whereas DE 1200 as well as the commercial product (Chd) were partially crystallized into cristobalite. The ability of the dust to release hydroxyl (*OH) radicals in the presence of hydrogen peroxide, as revealed by the spin-trapping technique, was as follows: Chd-F, DE 1200 > Chd > DE 900 > DE, suggesting that on heating, the surface acquires a higher potential for free radical release. Most of the silica samples generated COO* radicals from the formate ion, following homolytic rupture of the carbon-hydrogen bond, in the presence of ascorbic acid. A concentration-dependent decrease in cell proliferation and colony-forming efficiency was observed in SHE cultures treated with Chd-F, Chd, and DE. Heating abolished DE cytotoxicity but conferred a transforming ability to thermal treated particles. DE was the only sample that did not induce morphological transformation of cells. According to their transformation capacity, the samples were classified as follows: Chd-F > Chd, DE 1200 > DE 900 >> DE. Taken together, the reported results suggest that (1) the transforming potential of a biogenic amorphous silica is related to the thermal treatment that transforms the original structure in cristobalite and generates surface active sites; (2) the reactivity of samples in releasing *OH radicals correlates to their transforming ability; (3) the finer fraction of the commercial product is significantly more toxic and transforming than the coarse dust; and (4) opposite to silica dusts of mineral origin, which loose both cytotoxicity and transforming ability upon heating, heated diatomite acquires a cell-transforming potency. DE products should be thus considered a set apart of silica-based potentially toxic materials.
the substrate and limit the stress level, 2 air-cooling jets were added to the spray configuration to lower the coating temperature during deposition.Experiments were carried out considering 5 processing parameters: arc current (A), hydrogen ratio (B), total plasma gas flow rate (C), powder carrier gas flow rate (D) and injector diameter (E). The three first parameters are known to influence significantly the plasma jet properties (enthalpy, velocity, etc.) and the last ones influence particle trajectories in the plasma jet. A factorial design L 16 (2 5 ) was used to vary each of these parameters into 2 significant levels tacking into account all possible couple interdependencies as shown in Table 1.Coating friction behaviour characterization: Friction tests were performed using a Pin-On-Disc (POD) arrangement on a CSEM 1 tribometer. The POD test is a model test for determining friction characteristics and wear behaviors of two solid surfaces being in sliding contact. In this study, test configuration corresponded to a single point contact. Before performing the test, the samples were ground to lower their surface roughness (i.e., average roughness less than 1 lm). The coated sample were than slided against a 6 mm ball made of WC/Co under an applied load of 5 N. The sliding contact was maintained at 6 mm from the sample centre. The sample was rotated at 394 rpm corresponding to a linear speed of 0.33 m.s ±1 . The sliding distance was 1000 m reached after 51 min.
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