Traditional composites constructed from matrix and solid and/or porous fillers they feature high filler content and high hardness, their young modulus is dentin-like and their radioopocity is good. Depending on degree of milling. The surface is rough to nearly smooth. Depending on time and distrubition wear of the restoration and antagonists is low to high.
Andreas Mortensen,Concise encyclopedia of composite materials. P. 197
Polyethylene
Polyethylene, a linear polymer, is made by an addition reaction. It is started with an initiator, such as H2O2, which gives free, and very reactive -OH radicals. One of these breaks the double-bond of an ethylene molecule, C2H4, when it is heated under pressure, to give
The left-hand end of the activated monomer is sealed off by the OH terminator,
but the right-hand end (with the star) is aggressively reactive and now attacks
another ethylene molecule, as we illustrated in Fig. 22.1. The process
continues, forming a longer and longer molecule by a sort of chain reaction.
The OH used to start a chain will, of course, terminate one just as effectively,
so excess initiator leads to short chains. As the monomer is exhausted the
reaction slows down and finally stops. The DP depends not only on the amount
of initiator, but on the pressure and temperature as well.
Michael F. Ashby
David R. H. Jones Engineering Materials 2. An Introduction to Microstructures, Processing and Design. P. 280Nitrocarburizing
Nitrocarburized microstructures also develop in gaseous atmospheres when ammonia is blended with a carbon-emitting gas Gas nitrocarburizing is widely applied, with the use of additional gases such as endogas, exogas, carbondioxide , methylamine, and methanol resulting in several processing variants.
In nitrocarburizing, an ammonia atmosphere is blended with carbon and oxygen containing
gases that are usually based on carbon dioxide or allow carbon dioxide to be generated. In accordance with the homogeneous water gas reaction. carbon dioxide reacts under near equilibrium conditions with the hydrogen generated according to the react ion
CO2 + H2 --->H2O+ CO
Given a constant ammonia partial pressure, this leads to a drop in the hydrogen partial pressure and arise in the nitriding characteristic
A state of near-equilibrium is also maintained in the react ion of ammonia with carbon monoxide from an additional gas
NH3 + CO ---> HCN + H2O where hydrogen cyanide (HCN) is generated
In addition to reaction nitrogen transition is furthermore promoted by the reaction
HCN ---> N(a) +C(a) + (1/2)H2
in which carbon too diffuses into the steel. Carbon, moreover , diffuses into the steel via the Boudouard reaction and the heterogeneous water gas reaction
The Effect of nitrocarbuzing
George, E, Totten, Steel Heat Treatment Handbook Mettallurgy and Technologies. P. 458. 472-473. 477
Cryogenic Treatment ( Subzero Cryogenic Treatment)
Subzero cryogenic treatment may be applied to transform the retained austenite to martensite, substantially lowering its amount, sometimes to as little as about 1 vol%, which cannot be detected metallographically but only by x-ray diffraction
Decreasing the amount of retained austenite achieves
1. Increase in hardness and consequently in wear resistance
2. More dimensional stability in the finished part (smaller change in dimensions due to structural volume change in use)
3. Less susceptibility to the development of cracks at grinding
Figure 6.124 shows a heat treatment cycle that includes subzero treatment. The most
important parameters of the treatment are (1) the temperature below 0 C (328F) that should be attained and (2) the cooling capacity of the equipment. In some cases, temperatures of - 80 to -100 C ( -112 to - 148 F) are sufficient, but for other steels, especially high-alloy ones, ower temperatures of - 140 C (- 280 F) or even -180 C (- 292 F) are necessary. Holding time at low temperature is unimportant, because the transformation of retained austenite to martensite does not depend on time, but only on the temperature to which the metal has been cooled. Only that portion of the retained austenite will be transformed to martensite that corresponds to the cooling temperature realized. Further transformation will take place only if the temperature is lowered further.
George, E, Totten, Steel Heat Treatment Handbook Mettallurgy and Technologies. P. 400-401
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