Wettability And Spreading Dynamics Of Carbide Ceramics By Molten Metals

Carbides, because of their high melting points, hardness, and good thermal and chemical stabilities, are widely used in preparing metal-matrix composites, cemented carbides, wear-resistant coatings, etc. In these processes, the wetting between metal and carbide is often involved. The proper characterization of wettability and interfacial microstructure and the deep understanding of wetting mechanism and spreading dynamics can provide important basic data and theoretical guidance for the fabrication of materials, improvement of engineering technique as well as achievement of structural and functional parts with outstanding properties when the liquid phase is involved in.In the reactive wetting of metal/ceramic systems at high temperatures, dissolution, adsorption and reaction are usually involved. Most of the current researches focus on the latter two stages while more or less ignore the role of the first stage. In fact, dissolution, as an important mechanism for the wetting in some systems, cannot be neglected; On the other hand, the role of reaction and adsorption in determining the final wettability and their effect on the spreading dynamics are still controversial issues in current academia. In this dissertation, we introduced an improved sessile drop method, selected some characteristic systems, adopted high temperature interruption and selective corrosion techniques, and employed SEM, FESEM, EDS, XRD and XPS means to study the wetting mechanisms. Meanwhile, in combination with the theories of interfacial reaction thermodynamics and spreading dynamics, the roles of dissolution, reaction and adsorption on the wettability and spreading dynamics were revealed. The major results of the present study are as follows:(1) In Ni/Carbide (B4C, SiC, TiC, ZrC) systems, although the dissolution can improve the wetting, the effect is not very significant. The driving force for the dissolution is from the chemical potential difference of the solutes in the substrate and the melt at the solid/liquid interface, and that for the spreading is from the enrichment of the dissolved elements at the surface or/and the interface. The thermodynamic properties of the dissolved elements and the interfacial microstructure determine the final apparent wettability. The stronger the affinity of the dissolved element for the substrate (or the weaker affinity for the solvent metal) together with the more straight the interface, the smaller the final apparent contact angle is.(2) The wetting behavior involved in the metal/cabide systems driven by the dissolution is similar to that in the metal/metal systems and can be described by both molecular dynamic model and modified Jiang's empirical formula. However, the spreading velocity in the metal/carbide systems is usually slower than that in the metal/metal systems.(3) In the systems with distinct reactions, such as Al/B4C and Cu-Me/B4C, the nature and the continuity extent of the reaction products at the solid/liquid interface determine the final wettability, while the intensity of the reaction or the precipitation rate of the products directly affect the spreading dynamics. Generally spreaking, high reactivity accelerates the spreading rate and shortens the time required to reach an equilibrium.(4) The reaction-limited spreading needs a critical minimum concentration of the active elements in the melt. When the concentration is below the critical value, both the dissolution and reaction may influence the wetting and spreading (e.g., the warping of the triple line in the Cu-1at.%Ti/B4C system and the secondary spreading in the Ni-10at.%Ti/B4C system are the external evidences of this mechanism); When the concentration is above the critical value, both the adsorption and reaction may influence (e.g., the perfect wetting in the Cu-5at.%Ti/B4C system results from this mechanism) them.(5) The linear spreading in the reaction-limited spreading needs to satisfy the following conditions:1) The active element should reach a certain concentration at the solid/liquid interface; 2) The rate of the reaction should be comparatively slower than the suppling rate of the active element at the triple line; 3) The quasi-chemical equilibrium should be reached at the region close to the triple line; 4) The wettability of the reaction product by the molten metal is better than that of the original substrate.(6) In the Al/TiC system, the spreading can be driven by both the reaction and adsorption, and the latter plays an important role, which cannot be underestimated. Moreover, the wetting mechanism and the driving force for the spreading can be independent, i.e., the spreading can be controlled by the reaction or dissolution dynamics, while the wetting can be promoted by the adsorption of [Ti].(7) In the Zr55Cu30Al10Ni5/TiC system, the adsorption-promoted spreading can be described by the molecular dynamic model. The emergence of a precursor film is related to the adsorption-induced accumulation of the active atoms at the triple line. The formation of the precursor film requires not only a strong affinity of the active solute for the fresh substrate but also the absence of an intense reaction or dissolution at the solid/liquid interface.(8) Combining (6) and (7), important evidences were achieved in some systems that in some systems the adsorption can significantly improve wettability regardless of the source of the active element (i.e., externally added or dissolved from the substrate). These evidences support the viewpoint proposed by Saiz et.al that the spreading can be driven by the adsorption of the active element without requiring the precipitation of reaction product at the solid/liquid interface. In our viewpoint, both the adsorption and the formation of the reaction product can promote the wettability. The difference is that the relative role of the adsorption and reaction is different in a concrete system.(9) When the condition ofσlv(cosθe-cosθd)<<2nkT is satisfied, the molecular dynamic model, which can describe the adsorption-limited spreading, has the same expression as that of the reaction-limited spreading model. As a consequence, it is difficult to distinguish the role of the reaction and adsorption when both of them are involved in the spreading, even though their mechanisms for promoting the wetting are quite different.In conclusion, this study not only contributes to our understanding of the wettability and interfacial chemistry in the metal/ceramic systems at high temperatures, but also provides useful information and positive guidance for the preparation of the carbides reinforced metal matrix composites.