The effect of carbon nanolayers on wetting of alumina by NiSi alloys

Ionocovalent oxides such as alumina, silica or magnesia are not wetted by Si and Si-rich alloys, the contact angles being close to 90°. The aim of this work is to study the effect of submicron carbon layers on wetting in this type of system. In principle, silicon reacts with carbon to form silicon carbide, a compound wettable by Si alloys. However, the formation of silicon carbide at the interface can be affected by the dissolution of this compound into the molten alloy occurring in order to saturate the melt in carbon. These phenomena are studied using a model system consisting of Ni–63 at.%Si alloy and monocrystalline alumina substrate coated with carbon layers. Wetting experiments are performed by the dispensed drop technique in high vacuum varying the parameters: thickness of coating (from 0 to 100 nm), temperature and degree of carbon saturation of the alloy. The surfaces and reactive interfaces are characterised by SEM, X-ray microanalysis and XPS.     

Reactivity between liquid Si or Si alloys and graphite

The fundamental issues of the reaction at liquid Si/graphite interfaces between Si melting point (1412 °C) and 1600 °C are studied on the basis of results obtained with polycrystalline graphite concerning the growth kinetics of the interfacial reaction layer and the microstructure and morphology of this layer. Experiments were also performed using vitreous carbon substrates. Results are also reported for Si–Al alloys at 1000 °C. The elementary process controlling the growth kinetics is determined and a model is proposed to describe the different stages of the interfacial reaction.     

Experimental investigation on the enthalpies of formation of the DyFe2, DyFe3, Dy2Fe17, ErFe2, and ErFe3 intermetallic compounds

The enthalpies of formation at 298.15 K for five intermetallic phases in the Fe-Dy and Fe-Er binary systems have been determined by indirect solution calorimetry in liquid aluminum at 1100 K. The phases were Fe₂Dy, Fe₃Dy, and Fe₁₇Dy₂ in the Fe-Dy system and Fe₂Er and Fe₃Er in the Fe-Er system. The following mean values of Δ _f H _{298.15 K} are reported: −11.1 kJ/mole for Fe_2/3Dy_1/3, −7.7 kJ/mole for Fe_3/4Dy_1/4, −1.9 kJ/mole for Fe_17/19Dy_2/19, −12.5 kJ/mole for Fe_2/3Er_1/3, and −7.9 kJ/mole for Fe_3/4Er_1/4. The measured enthalpies of formation of the Fe₂Dy and Fe₂Er compounds are almost equal, as are the enthalpies of formation of the Fe₃Dy and Fe₃Er compounds. The results are compared with earlier experimental data, with values predicted by Miedema’s semiempirical method and with calculated results obtained by Colinet and Pasturel using a tight binding model.     

Brazing of AlN to SiC by a Pr silicide: Physicochemical aspects

In view of their very different thermomechanical properties, joining of metals to ceramics by brazing is usually performed by means of one or more interlayers. In a recent investigation AlN was chosen as interlayer material for brazing SiC to a superalloy. The aim of the present study is to determine an alloy with a high melting point (close to 1200 °C) enabling brazing of AlN to SiC. Two types of experiments are performed with a Si-17 at.% Pr eutectic alloy (T_m = 1212 °C): sessile drop experiments to determine wetting and brazing of AlN and SiC plates to determine gap filling. Experiments are carried out in high vacuum to promote deoxidation. Interfacial reactivity, joint microstructure and type of failure occurring during cooling are examined by optical and scanning electron microscopy.     

Influence of oxygen partial pressure on the wetting of SiC by a Co–Si alloy

In this investigation the influence of oxygen partial pressure P_O2 on the wetting of SiC by a Co–Si alloy was studied. Wetting experiments were carried out in argon with different oxygen contents (from 5 to 1000 ppm). The relationship between wetting and deoxidation of surfaces (SiC and Co–Si alloy) was investigated. Calculations were performed to evaluate the temperature range over which deoxidation is possible. These calculations are in agreement with the experimental results.     

Further insight into interfacial interactions in nickel/liquid Sn–Ag solder system at 230–350 °C

Interfacial reactions are investigated between electrochemical deposited Sn-2 wt%Ag alloy and Ni for isothermal heating at various temperature (230–350 °C) and for various time to study initial stages (1–4 min) and latter stages of reaction (15 min–4 h). During the isothermal heating a continuous compound layer of Ni₃Sn₄ is formed at the interface between liquid Sn–Ag and solid Ni. In this study scallop like morphology with round and smooth surfaces of Ni₃Sn₄ intermetallic (IMC) layer is observed for shorter time of isothermal holding, which is in fact contradictory to the observations reported by recent studies which describe the morphology of IMC as elongated and faceted needles. For longer reaction times (>1 h) the scallop-like morphology is transformed gradually to a facetted abnormal growth morphology but not elongated structure. The average thickness of the reaction layer is proportional to a power function of the annealing time with an exponent n varying from 0.35 to 0.40 and the apparent activation energy for liquid–solid Ni₃Sn₄ formation was evaluated to be of about 21 kJ mol^?1. The role of deposition method of Ni and Sn layers on the morphology and the growth kinetics of the reaction layer is discussed. A theoretical analysis of the initial formation and growth of Ni₃Sn₄ phase at the Ni/Sn interface is also presented.     

Factors governing interfacial reactions in liquid metal/non-oxide ceramic systems: Ni-based alloy–Ti/sintered AlN system

Wetting of ceramics by liquid metals is often promoted using alloying elements which form at the interface, by reaction with the ceramic, continuous layers of a better wetted compound. These layers can, in turn, improve or be detrimental to the mechanical performance of the interface, depending on their microstructure and thickness.The aim of this investigation is to determine the factors governing the growth kinetics in metal/non-oxide ceramic systems in which strong reactivity is often observed. The study system consists of a predominantly covalent ceramic, AlN, and a Ni-based liquid alloy containing Ti. Experiments are performed by varying the temperature, Ti content of the alloy and level of vacuum in the furnace. Point experiments were also carried out for a Au based alloy–Ti/AlN and AgZr/AlN couples.