Limiting the development of Al4C3 to prevent degradation of Al/SiCp composites processed by pressureless infiltration
Introduction
Spontaneous or pressureless infiltration of ceramic preforms by liquid metals offers the potential to be an alternative, simple and low-cost processing route for the manufacture of near-net shape silicon carbide (SiC) reinforced aluminum–matrix composites. However, there are still some barriers that have precluded its advantageous utilization as a commercial fabrication route. The typically low infiltration rates of ceramic preforms by molten metals without the aid of any external force result in long contact times between the ceramic reinforcement and the liquid metal. Therefore, in the case of the Al/SiC system, there exists the potential for the development of the harmful reaction product Al4C3 at the Al/SiC interface from the dissolution of SiC by liquid aluminum, according to [1], [2]The Al4C3 phase is unstable and reacts with water, according to the following possible reactions [3], [4]:The net result is a decrease in the properties of the composites, manifested either as a gradual (by the slow interaction with the atmospheric moisture) or prompt degradation of the material.
In spite of the number of technological approaches reported in the literature, the problem of the potential dissolution of SiC by aluminum remains unresolved, particularly when using the pressureless infiltration method. It has been demonstrated that silicon is a versatile element that can be used to control the reactions at the Al/SiC interface and prevent the potential attack of the SiC reinforcements by liquid aluminum [5], [6], [7], [8], [9], [10], [11], [12]. One of its major benefits is to reduce the activity of aluminum and ameliorate the potential damage caused by the molten metal on the SiC reinforcements. It can be used not only as an alloying element in aluminum, but also it may be present on the surface of the reinforcements as a coating either as an element or as an oxide. As a free-Si coating on the surface of SiC, it improves the wettability by significantly reducing the contact angle between the molten aluminum alloy and the substrate [5], [6].
A coating of SiO2 obtained by the passive oxidation of the SiC reinforcements promotes formation of new phases that are thought induce a strong bonding between the matrix and the reinforcement, and additionally provide stability to the composite. Depending on the content of Mg in the aluminum alloy and processing temperature, reactions for formation of MgO or MgAl2O4 in the composites may be favored [7], [8]. Through these reactions, SiO2 is practically used as a source of the silicon necessary to impede formation of Al4C3.
Due to its simplicity, in recent years the attention in the passive oxidation method as a means to prevent formation of Al4C3 during the fabrication of Al/SiC composites by routes with the metal in molten or semi-liquid state has grown substantially [7], [8], [9], [10], [11], [12]. However, there are inherent issues that might be prudent to consider if a processing route that promises to be economic and versatile is pursued. Fabrication of Al/SiC composites using SiC reinforcements artificially oxidized is in fact a two-stage route because passive oxidation can be considered as a pre-conditioning operation. It is in general a time consuming process. A typical oxidation routine involves heating SiC powders at rate of 10 °C/min and hold to preset temperatures in the range from 1100 to 1300 °C for 40 h [9]. As an example, a layer of SiO2 with a thickness of 100 nm can be formed on the surface of SiC at 1100 °C for 2 h and a 1000-nm-thick layer may be produced at 1300 °C during 40 h [9], [10].
A major disadvantage in the passive oxidation method is the difficulty to control the thickness and uniformity of the layer formed, particularly when oxidizing a bed of powders. Not only is important to control the layer thickness for economic reasons, but also because the strength of the interface depends on the thickness of the reaction products formed, such as MgAl2O4 [9]. Moreover, depending on the crystal structure of the SiO2 layer, volume changes may occur during heating when fabricating the composites. For instance, in the range of temperatures 220–275 °C, an expansion accompanies conversion of α-cristobalite to β-cristobalite; transformation of α-quartz to β-quartz involves a volume increase of approximately 1.6% [13]. These volume changes may result in the fracture of the SiO2 layer and in a decrease of the reinforcement/matrix interface strength. Since the liquid metal can diffuse through the microchannels formed during the layer fracture, ultimately SiC is attacked and dissolved by aluminum. Hence, a layer of SiO2 formed by intentional oxidation of SiC does not completely inhibit formation of Al4C3.
The thickness of the SiO2 layer also depends on the particle size of the SiC powders. In the work by Ureña and co-workers [11], analyses by thermogravimetry (TGA) showed that while a layer of only just 50 nm is formed in powders of 18 μm particle size, a layer of 160 nm is formed in powders of 43 μm at an oxidation temperature of 1500 °C. At 1200 °C, a layer with a maximum average thickness of 93 nm is achieved during 8 h and one of 55 nm is obtained after treatment for 2 h.
It is also important to recognize, that during these operations, non-friendly with the environment gases are liberatedThus, formation of a layer of SiO2 that apparently will protect SiC might be favored at the expense of pollution of the environment. Furthermore, the SiC reinforcements may be consumed during the oxidation operations according to [9]Instead of oxidizing the SiC reinforcements, the authors are currently performing an investigation with the aim of limiting or retarding formation of Al4C3 by the incorporation of silicon dioxide particles (SiO2p) into SiCp preforms, during the processing of Al/SiCp composites by the non-assisted infiltration route. After certain time during the liquid metal infiltration of a porous SiCp-preform, a metal/composite interface is created. Since the outer surface of the preform is the area exposed to the liquid aluminum the longest time period, it is expected to be more susceptible to the formation of the harmful aluminum carbide (Al4C3) phase. Consequently, the soundness of the composite depends in a large extent on the condition of the metal/composite (M/C) interface.
In this work, the effect of the presence of 6 vol% SiO2p into SiCp preforms and the SiO2 crystal structure on the microstructure of the composites and the metal/composite interface was investigated. In addition, as a means to test the efficacy of this alternative method to prevent formation of Al4C3, specimens were deliberately exposed to the environment at room temperature and the composites as well as the metal/composite interfaces were examined with time.
Section snippets
Experimental
Commercial SiCp and reactive grade SiO2 powders were used for preparation of three types of preforms. In Table 1 the alloy/reinforcement systems used in the experiment are shown. For systems II and III, the SiCp were mixed thoroughly with 6 vol% of SiO2 powders. In addition, 10 wt% dextrin and approximately 0.5 ml of distilled water was used to prepare the preforms for all the three systems. Then, the mixtures were placed in a steel die and compacted to produce cylindrical preforms (2.0 cm high × 2.0
Results and discussion
Results from characterization of the SiC and SiO2 powders using a Coulter LS particle size analyzer and a helium gas picnometer, are shown in Table 2.
It was observed that the higher the temperature and the longer the test time, the higher the tendency for Al4C3 formation. Accordingly, with the aim of illustrating the effect of incorporating 6 vol% of SiO2 powders into SiCp preforms, in this paper results only from the analysis of specimens obtained under the most severe conditions (1100 °C for 60
Summary and conclusions
Results from this investigation reveal that the presence of 6 vol% of SiO2p in the Al–Mg–Si/SiCp system has a beneficial effect since it prevents or retards the development of the harmful Al4C3 phase. The absence of Al4C3 in systems II and III is explained in terms of the feasibility of the reaction for the formation of MgAl2O4 and the Si supplied to the system. Analysis by EBSD reveals that irrespective of the SiO2p crystal structure, all the silica incorporated into the SiCp preforms reacts to
Acknowledgments
Authors gratefully acknowledge CONACyT for financial support under contract 34826-U. M. Rodríguez-Reyes also expresses his gratitude to CONACyT for providing a scholarship. Finally authors thank Microabrasivos de México S.A. de C.V. for supplying the SiC powders and Mr. Felipe Márquez Torres for assistance in the microscopic analysis.
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