Wetting Of Ceramics By Molten Mg And Effect Of Evaporation

In recent decades, magnesium and its alloys are potential materials for advancedstructural applications such as in automobile and aerospace industries with significantadvantages of low density, high specific strength and high stiffness in combination with highdamping capacity, good castability and machinability. However, the characteristics of theirsmall elastic modulus, low strength at elevated temperatures and poor wear and corrosionresistance hamper their applications. In order to overcome these shortcomings, considerableefforts have been made to develop the magnesium matrix composites reinforced with variousceramic particles such as Al₂O₃, SiC, B₄C, TiC and TiB2. Therefore, the development ofmagnesium matrix composites is a promising way to broaden their applications.It is well known that the wettability of ceramics by molten metals plays a crucial role inpreparation of metal–matrix composites using a liquid casting or infiltration route. Theinterfacial chemistry also, to a large extent, determines the bonding quality of thecomponents in the composites. Therefore, it is of vital importance to understand and furthercontrol the wettability and interfacial chemistry between the matrix and the reinforcement.However, because of ready oxidation of Mg at relatively low temperatures and extensiveevaporation at high temperatures, an accurate measurement of the wettability for Mgbecomes rather difficult. To our knowledge, so far, only limited work has been performed onthese important issues for molten Mg. In the present study, we studied the wetting andinterfacial characteristics for molten Mg on oxides, carbides and nitrides by an improvedsessile drop method, and presented the clear modes and laws for the wetting underevaporation. Meanswhile, in combination with the relationship between wetting andevaporation dynamics, the methods to evaluate the wettability and the complex wettingbehavior were proposed. On the other hand, we provided some references to the selection ofthe reinforcement for Mg–matrix composites from a viewpoint of wettability and chemicalstability. The major results of the present study are as follows:(1) The intrinsic wettability of Mg/oxides (MgO,Al₂O₃,ZrO₂,SiO₂) systems wasobtained. The initial contact angles are in the ranges of90–76°,94–67°,117–90°and56–35°(973–1073K) at973–1173K, respectively, decreasing with increasing temperature. In viewof reactivity, Mg–MgO is an inert system, Mg–Al₂O₃and Mg–ZrO₂are weak–reactionsystems, while Mg–SiO₂is a strong–reaction system. For the systems involving a volatiledrop, the role of the chemical reaction in improving wetting, as shown in the Mg–Al₂O₃andMg–ZrO₂systems, is more or less limited, which is mainly embodied in the initial stage, andthen is shielded or weakened by the evaporation; while in the Mg–SiO₂system, the chemicalreaction greatly promotes the wetting. (2) The intrinsic wettability of Mg/caibides (SiC, B₄C, TiC), Mg/graphite (C) andMg/nitrides (Si₃N₄and AlN) systems was also obtained. The initial contact angles are in theranges of83–76°,95–87°,74–60°,142–124°,80–60°(973–1123K) and75–36°(973–1123K)at973–1173K, respectively, decreasing with increasing temperature. From the viewpoint ofinterfacial bonding, Mg/SiC, Mg/B₄C, Mg/TiC, Mg/Si₃N₄and Mg/AlN systems showchemical nature while Mg/C system physical.(3) Two typical modes for the wetting under evaporation are identified:(i) constantcontact radius with diminishing contact angle;(ii) constant contact angle with diminishingcontact radius. The former is observed in all the systems with relatively rough substratesurface, while the latter only on a smooth surface. The concrete wetting mode is closelyrelated to the roughness of the substrate surface and the degree of oxidation of the Mg dropsurface. For a reactive system, the accumulation of reaction product at the triple line hindersthe movement of the triple line, thus strengthening the constant contact radius (the pinning ofthe triple line) mode.(4) The distinct "stick–slip" behavior during evaporation was explained by thecompetition between the excess free energy stored within the system and the hystereticenergy barrier opposing the movement of the triple line. The larger the surface roughness orthe heterogeneities of the substrates, the more the energy required for overcoming thepotential barrier. When the excess free energy exceeds the potential energy barrier, the tripleline de–pins and recedes. The receding time, frequency and magnitude are stronglydependent on the surface roughness of the substrate rather than on temperature.(5) The criterion for the evaluation of the wettability and the wetting dynamic under theevaporation has been proposed. For the system involving a volatile drop, the wettability isbetter to be characterized by using the initial contact angle and the wetting behavior to beassessed by combination of the changes in the contact angle (θ), contact radius (r) and dropheight (h). The true wetting improvement must satisfy the conditions of dθ/dt<0and dr/dt>0.For the systems with concomitant evaporation and reaction, the wetting behavior is better tobe assessed by the changes in the hypothetic contact angle (θ^*, see Section3.3.1) and theactually measured contact angle (θ). The relative contributions of the spreading and theevaporation to the decrease in the contact angle are characterized by (θ₀-θ^*)/(θ₀-θ_f) and(θ^*-θ)/(θ₀-θ_f), respectively.(6) The wettability of Mg/(110)MgO and Mg/(111)MgO systems is better than that ofMg/(100)MgO system, while the pinning of the triple line on the (100)MgO face is weakerthan that on the other two faces. In addition to the substrate orientation, the wetting ofceramics by molten Mg may be affected by some external factors, such as substrate surfaceroughness, gas flow rate and temperature. For the Mg/α-Al₂O₃system, the wettability doesnot depend on the Al₂O₃substrate orientation. The final reaction product in the reaction zoneconsists mainly of MgO and MgAl₂O₄phases together with displaced Al. The epitaxialgrowth of MgO and MgAl₂O₄°n α-Al₂O₃shows the following relationships: C(0001)_Al2O3//(111)_MgO,R(101|-2)_Al2O3//(100)_MgO//(100)_MgAl2O4,M(101|-0)_Al2O3//(110)_MgO//(110)_MgAl2O4。(7) In view of the wettability and chemical stability, this study offers the followingguidance for preparation of the magnesium matrix composites:â‘ MgO and Al₂O₃have good wettability with molten Mg and thus they are expected tobe the desirable reinforcements for the Mg–matrix composites. Although the wettability ofZrO₂by molten Mg is poor, the chemical reaction leads to the formation of MgO phase atthe interface, which improves the wettability to some extents and makes the ZrO₂acandidate material of the reinforcement. SiO₂has good wettability and strong interfacialreaction with molten Mg, and thus it may be used as a reaction agent for preparing theMg–matrix composites reinforced by in–situ MgO–Mg2Si particulates.â‘¡Among carbides, SiC and TiC have good wettability and chemical stability withmolten Mg and thus they are expected to be the desirable reinforcements for the Mg–matrixcomposites. The wettability of B₄C and C by molten Mg is poor and thus they are not idealreinforcements.â‘¢AlN and Si₃N₄share good wettability and interfacial bonding with molten Mg andthus they are suitable reinforcements.In a word, this study not only enriches our understanding of the fundamental aspects ofthe wetting and interfacial chemistry in the Mg/ceramic systems at high temperatures butalso provide guidance for the preparation of the ceramics–reinforced Mg matrix composites.