Wetting Of Three Ceramic Substrates Normally Used As Reinforcements By Molten Mg-Al Alloys

Ceramic reinforced magnesium-and aluminum-matrix composites have received great attention because of outstanding properties such as low density, high specific strength and stiffness. As we know, when a liquid casting or infiltration route is employed in the fabrication of metal matrix composites, the wettability between metallic matrix and ceramic reinforcement largely determines the ease of the process, the bonding quality and the final properties of the composites. On the other hand, the evaporation is ubiquitous for liquid metals especially the volatile substances such as Mg at high temperature, and most of ceramics can react with molten Mg-Al alloys. For a material system under the circumstance of the coupling of wetting, evaporation and reaction at high temperature, how to evaluate the intrinsic wettability and wetting kinetics is an important basic scientific topic.However, Mg is highly volatile, and Mg-Al alloy is easily oxidized and also easy to react with ceramics, which would lead to difficulty in the wetting experient. As a result, up to date, the understanding of the wettability and interfacial bonding characteristics between molten Mg-Al alloy and ceramic is very poor, which could hinder the development of magnesium-aluminum matrix composites. Therefore, in this paper, we investigated systematically the wetting of three ceramic substrates normally used as reinforcements (Al2O3, SiC and SiO2) by molten Mg-Al alloys over a full composition range using an improved sessile drop method, and examined thoroughly the interfacial microstructures of some cooled samples as well as their behavior of evolution by using XRD, SEM-EDS and other means. Furthermore, the interfacial reaction was analyzed deeply by combining thermodynamic calculations and experimental results. The main studies obtained are as follows:(1) A scientific criterion for evaluation of the wetting behavior under the circumstance of the coupling of evaporation and reaction was proposed. The initial contact angles measured by improved sessile drop method were suggested to be used to characterize the intrinsic wettability of the original systems. The characterization of wetting kinetics should simultaneously concern the variation in contact angle θ and contact diameter D (or radius R) with time:(i) dD/dt>0, d0/dt<0, suggesting that the interfacial reaction promotes the spreading of the triple line; the contact angles are advancing angles and the wetting is really improved.(ii) dD/dt=0, dθ/dt<0, suggesting that the triple line is pinned; the contact angle decreases due to the evaporation; the contact angles are receding angles and the wetting is not actually improved,(iii) dD/dt<0, suggesting that the triple line recedes; the change (increase, decrease or unchange) in contact angle depends on the competition between the recession of the triple line and the evaporation.(2) The common mechanism for the effects of Mg on the wettability in the Mg-Al/(Al2O3, SiC and SiO2) systems was revealed. Mg can decrease not only the surface tension of the A1drop (σ/ν) but also the solid-liquid interfacial tension (σsl). Provided that the drop surface is clean (non-oxidation), if the oiginal system is non-wetting (θ>90°), the addition of Mg does not always improve the wettability; the improvement of the wetting observed is usually attributed to the disruption of the oxide film on the Al drop surface caused by the Mg evaporation. Conversely, if the original system is wetting (θ<90°), Mg can significantly improve the wettability of the system.(3) The controlling mechanism for the movement of the triple line at high temperature was revealed. The trend of the triple line movement (spreading or receding) mainly depends on the difference in the instantaneous apparent contact angle [θa(t)] and the instantaneous assumed equilibrium contact angle [θe(t)]-If θα(t)>θe(t), the drop tends to spread; if θa(t)<θe(t), the drop tends to recede. The movement of the triple line was cotrolled by the competition between the driving force for the spreading or recession [Fd(t)] and the pinning force [Fp(t)]. If Fd(t)>Fp(t), the triple line moves. Conversely, the triple line is pinned. Fd(t) could be written as Fd(t)=σlv,(t)[cosθe(t)-cosθa(t)],σlv(t) is the instantaneous liquid surface tension; Fp(t) arises mainly from the physical (such as roughness) and chemical (such as chemical heterogeneities) defects of the solid surface.(4) The underlying relationships between wetting, evaporation, interfacial reaction and Mg-Al alloy concentration were established. The evaporation can not only reduce the drop volume and instantaneous apparent contact angle [θa(t)], but also change significantly the alloy concentration, thereby affecting the wetting. The interfacial reaction can influence the interfacial condition, the alloy concentration, and thus the wetting. The variation in alloy concentration could affect the wetting and the evaporation rate of the drop; in addition, it would influence the interfacial reaction, especially the formation of reaction products, and further make an effect on the wetting. Furthermore, the relationship between the interfacial reaction and the Mg-Al alloy concentration was established quantificationally based on thermodynamic considerations.(5) The underlying mechanism for the recession of the triple line in the Mg-Al/Al2O3 system was revealed. The recession of the triple line could be roughly classified into two cases. The first occurred in the stage with a decreasing drop volume and was mainly attributed to the increasing Al concentration in the alloy, which increased [θe(t)], and the diminishing drop volume, which decreased [θa(t)], resulting from the Mg evaporation; while the second in the stage with a constant drop volume and to the formation of MgAl2O4at the interface, which increased [θe(t)], and the evaporation of small amount of residual Mg in the drop, which increased [σlv(t)].(6) The underlying mechanism for the drop spreading in the Mg-Al/SiC system was revealed. The initial spreading was mainly controlled by the deoxidation of SiC substrate surface. At1173K, for the Mg-Al alloys with lower Mg concentrations such as8,20and43mol.%on the a-SiC substrate surface, the spreading showed a transition from slow to fast, which was primarily attributed to the segregation of Mg at the solid-liquid interface and the larger deoxidation rate of SiC substrate surface for Mg than Al.(7) The underlying mechanism for the spreading of the drop and the formation mechanism of the interfacial structures in the Mg-Al/SiO2system were revealed. The spreading was mainly attributed to the formation of Mg2Si at the interface, especially at the triple junction. The spreading rate increased with increasing the Mg concentration. Interfacial reaction product zones exhibited characteristic layered structures. The formation of these complex structures was controlled by the variation in instantaneous alloy concentration due to the evaporation and reaction from the viewpoint of thermodynamics and by the penetration and diffusion of Mg, Al and Si from the viewpoint of kinetics.The above results are expected to not only provide helpful guidance for the preparation and development of the magnesium-aluminum matrix composites, but also further enrich the scientific understanding of the wetting and interfacial chemistry in the metal-ceramic systems at high temperatures.