Thus, considering wetting as the main precondition for the formation of a solid structure, all metal composites should be divided into two main groups:
- metal composites, wherein the non-metallic phase is wetted by a molten metal well;
- metal composites, wherein the wetting of a non-metallic phase by a molten metal is not too much .
The composites of metal-bond metal-like refractory compounds, i.e. most carbides and silicides having the interstitial alloy structure and also borides and silicides, are referred to the first group. Due to its good wetting by molten metals a tight and stable bond between the metal and metal-like phase is typical of the metal composites of this type. Such metal composites sinter at the temperature that is a little higher than that of binder metal melting point. The formed at that fluid metal phase wets the surface of the metal-like compound searching into the finest cracks and its grains´ surface irregularities and providing thus the highest strength of the composite. At that, the formation of some carrier compounds and subsolution is possible.
The second group combines metal composites on the base of oxides. The last ones are wetted by the molten metals not properly, hence, to reach a stable bond between the metal and non-metallic component is not possible, as a rule. For this reason at sintering of metal composites of the oxide - metal compositions at the temperature exceeding the binder metal melting point the metal runout occurs. To avoid such a phenomenon the metal composites of the oxide type are burnt at the temperatures, which are lower than those of the metal´s melting point, at which the metal is in a plastic, but still solid state.
For the wetting quality improvement between refractory metals and high refractory oxides in the process of metal composites´ sintering the oxides should have a high-surface area. Sometimes the oxides forming together with the basic one the solution phase are introduced into the metal ceramic composition. The additions of its oxide are made to the metal, if this oxide can interact with the basic one of the system. The alloying additions are made to the basic metal; at that, the alloying metal should be an oxygen hungry one, that promotes the oxide´s wetting by the alloy being formed.
The oxide oriented metal composite will be strong and tight when the metal and non-metallic component form an intermediate layer connecting both phases. Such an ideal case is the introduced oxide and binder metal one solid-solution formation, for example, cermet of the corundum - chrome composition.
To a certain extent the properties of metal composites are determined by a range of physical-chemical processes taking course when heated:
- wetting of the ceramic phase by the metal;
- chemical interaction of the phases;
- mutual solubility of the phases .
The properties of metal composites can be changed when choosing different metal-non-metallic components relations, that is easy to achieve by powder technique practice.
When melting during the burning-in, the metal particles of the metal composite act as a binder wetting and connecting the grains of the other component or making a chemical compound with it. As the result of their interaction new materials possessing specific properties and not being simply a sum of metal´s and non-metallic component´s properties appear .
For the time being the formulation of metal composites is to a large extent selected empirically, but still there are some theoretical preconditions. The determinative factor in the binding material choice is its ability to wet the non-metallic component. The wetting can be attended in the metal composite by a chemical change between the components (they are the phases as well) with the formation of a new phase in the form of a new compound or a solid solution. The better the wetting, the higher the quality of the metal composite obtained.
In the sphere of obtaining composite materials high temperatures most of the components are thermodynamically nonequilibrium ones, able to undergo different reactions with each other at the phase boundary. Physical-chemical phenomena taking course on the components´ boundary surface are rather complex and nowadays studied not well enough. Generally, the accumulation of experimental data on the interaction between the most advanced metals (aluminum, magnesium, nickel, titanium and some others) and fibers - boracic, carbon, glass. But it should be noted that nowadays there is some calm in composite materials studying, and the top of the investigations falls on 70-80s when the majority of the data were got and some theoretical developments were started both in our country and abroad.
- Chemical technology of ceramics and firebricks. // Under the reduction of Budnikov P.P. - M.: Construction literature publishing house - 1972, p. 552.
- Korelova A.I. Glass, ceramics and their future. - Leningrad: Higher School - 1962, p.56.