Papers by Author: Martin I. Pech-Canul

Abstract: The degradation in ambient atmosphere of Al/SiCp composites prepared by the reactive infiltration of SiCp preforms containing fly ash has been investigated. SiCp/fly-ash preforms in the form of plates (3 cm x 4 cm x 0.5 cm) with 50 % porosity are infiltrated by an Al- 8 Si-15 Mg (wt. %) alloy under argon atmosphere at 1050, 1100 and 1150 °C, for 50, 60 and 70 min. Characterization by XRD, SEM and EDX of composite specimens shortly after processing do not reveal the presence of the unwanted Al4C3 phase. However, in addition to Al, Si and SiC, MgAl2O4 and Mg2Si phases are detected. One month after the infiltration trials, white and gray powders are present on the composite specimens, accompanied by pitting corrosion and cracks which propagate with time. Although analysis by XRD of the degradation products reveals only Al4C3 in addition to the above mentioned phases, results from SEM, IR absorption and ICP also suggest the presence of Al(OH)3 and Mg(OH)2, probably from the interaction of Al4C3 and Mg2Si with water. It is considered that Mg2Si in the powders acts as an anode in a galvanic couple with atmospheric moisture as the electrolyte. The crack pathway through SiC, intermetallic AlFeMnSi and Si rich zones implies that one or more of these phases worked as the cathode. In summary, degradation of the composites is explained by the combined effect of galvanic corrosion caused by second phases and the interaction of Al4C3 with atmospheric moisture.

Abstract: The effect of particle size distribution and particle size ratio of SiCp in SiCp/SiO2 preforms on the microstructure, microhardness of SiCp reinforcements, modulus of rupture, and superficial hardness of Al/SiCp composites produced by pressureless infiltration has been investigated. SiCp/SiO2 preforms in the form of plates (4cm x 3cm x 0.5cm) have been pressureless infiltrated by the alloy Al-15.52 Mg-13.62 Si (wt. %) at 1100 oC for 60 min under inert atmosphere. SiC powders with average particle size of 10, 68 and 140 μm are mixed with SiO2 powders and preforms of 40 % porosity with unimodal, bimodal and trimodal size distributions are prepared by uniaxial compaction. The bimodal (small: large) and trimodal (small: medium: large) preforms are prepared with different particle size ratios in the following levels: 1:1, 3:1, 1:3, 2:2:2, 3:2:1, 3:1:2. Results from characterization by XRD, SEM and energy dispersive X-ray spectrometry show that the typical microstructure of the composites contains the MgAl2O4 (spinel), AlN and MgO phases formed during processing as well as partially reacted silica, SiC, Si and Al. It is found that the density, reinforcement microhardness, modulus of rupture and superficial hardness of the composites increase all with wider particle size distribution. However, whilst the modulus of rupture is mainly affected on going from unimodal and bimodal to trimodal distribution, superficial hardness and microhardness are mostly influenced on going from unimodal to bimodal and trimodal distribution.

Abstract: The effect of processing parameters on the weight loss of the silicon solid precursor (Na2SiF6) and the deposition characteristics and morphology of Si3N4 formed onto SiCp/Si porous substrates by CVD has been investigated. The results show that the weight loss of Na2SiF6 is most significantly affected by the processing temperature, followed by the processing time and the type of nitrogen precursor. Formation of Si3N4 is mostly influenced by the substrate temperature, followed by the type of nitrogen precursor and processing time. An increase in processing time and temperature from 60 to 120 min and from 900 to 1300 oC, respectively, favors dissociation of Na2SiF6 and formation of Si3N4. Moreover, N2 enhances Na2SiF6 dissociation and hampers Si3N4 formation, while the N2-NH3 mixture hinders the solid precursor dissociation and favors Si3N4 formation. With regard to microstructure evolution, it is found that in N2 the amount of Si3N4 increases with temperature and the morphology changes from wool-like and light fibers to thicker and compact fibers. When N2-NH3 is used and the processing temperature is increased, the morphology of Si3N4 is modified from deposits with wool-like and compact appearance to whiskers and spheres and finally to thick and compact fibers.

Abstract: CVD silicon nitride (Si3N4) is typically produced from gas or liquid precursors containing nitrogen and silicon. The method using Na2SiF6(s) as silicon solid precursor to produce films/coatings, reinforcements and powders of silicon nitride by CVD has been recently proposed in the literature. In this investigation, a thermodynamic study is carried out using the FactSage Thermochemical Software and Databases, in order to explain the phenomena associated to the synthesis of Si3N4 with Na2SiF6 as solid precursor. Accordingly, CVD diagrams for Na2SiF6, SiF4, SiF3, SiF2, SiF, and Si both with N2 and NH3 are constructed using such a software. Thermodynamically Si3N4 can be produced from SiF4(g) or Na2SiF6(s) with ammonia. Although thermodynamic considerations show that Si3N4 cannot be produced with the use of nitrogen, experimental results in this investigation show that it is formed with both ammonia and nitrogen.

Abstract: The interfaces formed between vitreous or thermally devitrified fused quartz substrates and silver alloys after 90 min at 850 °C in vacuum have been characterized. Three silver alloys have been used: Cusil (Ag–28 wt % Cu), Cusil-ABA (Ag–35 wt % Cu–1.5 wt % Ti), and Incusil-ABA (Ag–27 wt % Cu–12 wt % In–2 wt % Ti). A non wetting condition is found for the Cusil alloy in both substrates. In contrast, the formation of Ti5Si3, Cu3Ti3O and Ti2O3, following the sequence SiO2 → Ti2O3 → Ti5Si3 → Cu3Ti3O, is observed at the metal/ceramic interface for the two titaniumcontaining alloys on both substrates. Ti2O3 is commonly found as small particles dispersed in a silver-rich matrix. During the experiments, the reaction product layers detach from the ceramic surface and float away from the ceramic/metal interface due to their relatively low density with respect to the liquid alloy. The formation of the phases detected at the ceramic/metal interface can be explained in terms of their relative thermodynamic stability.