The effect of surface roughness on early in vivo plaque colonization on titanium

This study assesses in vivo the surface roughness necessary to reduce plaque colonization on titanium after 24 hours. Three groups of 16 titanium disks were assigned to 3 different polishing groups (A, B, and C). The roughness was evaluated with a laser profilometer and the morphology with a scanning electron microscope (SEM). Eight volunteers were enrolled and two stents were applied in the mandibular posterior region of each. Each stent supported 3 disks, one per group. The volunteers suspended oral hygiene for 24 hours, after which the stents were removed; one was processed for evaluation of the adherent biomass and the other for SEM study. On each specimen a global area of 100 x 125 \u3bcm was examined with SEM. The area was composed of five 20 x 25 \u3bcm randomly selected fields. For each field the density of bacteria and the morphotypes were recorded. The data quoted for the global area are cumulative of those observed in the 20 x 25 \u3bcm fields. Group A had a significantly smoother surface than groups B and C. The adherent microbial biomass determination and SEM evaluation revealed that group A contained less bacteria than the roughest group. The bacterial population was composed of cocci in group A, and of cocci and short and long rods in groups B and C. We conclude that a titanium surface with Ra 64 0.088 \u3bcm and Rz 64 1.027 \u3bcm strongly inhibits accumulation and maturation of plaque at the 24-hour time period and that such smoothness can be achieved in transgingival and healing implant components

Stress corrosion cracking: Characteristics, Mechanisms and Experimental study

Stress corrosion cracking (SCC) is a phenomenon in which the cracking of a metal alloy usually results from the combined action of a corrodent and tensile stress. Stresses that cause cracking can be residual or may be applied during service. A degree of mechanistic understanding of SCC will enable most metallic engineering materials to operate safely though stress corrosion cracking failures still continue to occur unexpectedly in industry. In this paper, the characteristics, mechanisms and methods of SCC prevention are reviewed. The results of experimental studies on alpha brass are also reported of which the failure mode conformed with the film-rupture and anodic dissolution mechanism

Paesaggio rurale: stratificazioni, relazioni ed elementi nascosti [Rural landscape: hidden stratifcations, relationships and elements]

La ‘naturalezza’ di un sistema può essere definita come inversamente proporzionale alla ricezione di energia utile al mantenimento delle condizioni del sistema stesso. Più ci si allontana da una condizione ad energia zero più il sistema diventa artificiale. I processi e le attività umane invadono il territorio nascondendo spesso le sue profonde alterazioni, rendendo percepibile al contrario una irreale naturalità. Il paesaggio agricolo è un sistema complesso che esterna solo parzialmente i rapporti con gli elementi antropici che silenziosamente lo plasmano. Questo articolo sottolinea come il territorio rurale sia, nel suo intimo, profondamente artificiale e come la sovrapposizione delle sue componenti generi paesaggi complessi e stratificati

WED machinability of a 25% silicon carbide particle reinforced 6061/Al alloy composite

Wire Electro Discharge Machining is used for the processing of heat treated high quality alloys or for the machining of complex shapes of advanced materials. Results on the machinability performances of a 25% SiC particle reinforced Al 6061 T6 composite and of the Al 6061 alloy by both roughing and finishing methods are given. Machining tests were performed with the following parameters: 1) peak current 16, 32, 64 A; 2) pulse duration 1.6, 6.4 ms; 3) voltage 120 V; 4) wire SW25X, 0.25 mm diameter, Charmilles Technology. The machined surfaces were observed at the SEM using both secondary electrons (SE) and back-scattered electrons (BSE). EDAX semi-quantitative analyses were also carried out. The surface features, roughness, hardness and contamination for the process parameters used are pointed out. Results are compared with the ones obtained machining the base Al 6061 T6 Alloy

L'USURA AD ALTA VELOCITA'

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On the perfoemance of a CRN coated SIC whiskers reinforced ceramic tool high speed machining of NI-base super-alloys

The main limitation on the wider use of Nickel-base super-alloys, such as INCONEL 718 (UNS N07718) is the difficulty in conventional-type machining. Even the expensive SiC whiskers reinforced ceramic tools, that are more suitable for use at high cutting speeds, have a short life-time. The aims of the study is to obtain an increase in tool life by minimising the temperature effects on composite reinforcement mechanisms. 20% SiC whiskers reinforced Al2O3 tools were CrN coated by PVD. Comparative machining tests on INCONEL 718 were performed using 20% SiC whiskers reinforced Al2O3 tools and CrN coated 20% SiC whiskers reinforced Al2O3 ones. After 40 and 60×10-6 m3 alloy removal the tools were observed at SEM and EDAX semiquantitative analyses were carried out. The CrN layer behaviour under the various cutting conditions used was analysed. Different wear mechanisms along tool chip contact length were observed. The causes and the mechanisms of wear were deduced and a mathematics model linking tool life with process parameters is suggeste