## Key Variables Affecting Composite Production Using the Infiltration Method

Introduction

Some monolithic materials that are inadequate in meeting the rapidly increasing demands of modern technology are gradually ceding their positions in these application areas to composite materials. Composite materials constitute a field where intensive research is being conducted and the process of developing materials with new compositions and designs is continuously ongoing [1-9]. Intensive work is being carried out with diverse objectives such as finding new application areas for metal, ceramic and polymer matrix composite materials, expanding existing application areas, achieving better properties and realizing production in a more economical manner [10-16]. Numerous different production methods are used for the production of composite materials, ranging from the simplest to the most complex production techniques [17-22]. The infiltration method is one of the most widely used methods in composite production. In ceramic-metal composite materials, the properties obtained vary significantly depending on the ceramic and metal ratios in the composition. As the ceramic volume fraction in the composite increases, wear resistance and high-temperature strength also increase. Numerous different techniques can be applied for the production of composite materials containing reinforcing ceramic at low volume fractions. However, for the production of dense composite structures with high ceramic volume fractions, metal infiltration into pre-shaped porous ceramic pellets is one of the most suitable and limited number of methods available [23]. The infiltration of liquid metal into the porous ceramic structure can be achieved with the aid of pressure or vacuum, or alternatively, when liquid droplets wet the solid, the liquid can self-infiltrate into the porous structure without any external influence [24, 25]. Infiltration is generally examined under three different categories based on its driving force, as shown in Figure 1. The infiltration of liquid metal into porous ceramic material does not occur easily, largely due to insufficient wetting. Wetting can be simply defined as the ability of a liquid to spread over a solid surface. For the production of ceramic and metal matrix composites through infiltration processes, the liquid metal must wet the ceramic phase. Perfect wetting is achieved when the contact angle is 0°, whereas when the wetting angle is 180°, the system becomes completely non-wetting. Generally, the contact angle between metal oxides and metals is greater than 90°. For this reason, external pressure must be applied to enable liquid metal infiltration into the capillary voids and rough surfaces present on the solid surface [25]. The pressure application process can be applied with the aid of a gas or mechanically.

It is known that working under vacuum during the infiltration process aids the infiltration of molten metal into the ceramic pellet. In some metal-ceramic systems, it has been observed that creating a vacuum environment around the ceramic phase is sufficient to create a pressure difference of a magnitude that enables infiltration.

Additionally, it is reported that in the infiltration process under vacuum conditions, oxidation of the liquid metal is prevented and the formation of pores is prevented due to gas or air remaining in the composite structure being produced. The infiltration method conducted without external pressure application, with specially designed composition or process conditions and good wetting, is a simple and economical method that allows the production of complexly shaped products in a very dense manner with virtually no shrinkage observed. In this method, liquid metal self-infiltrates into the porous ceramic body without pressure application and initially fills the voids in the porous ceramic pellet and subsequently solidifies in place. One of the most important advantages of the infiltration method is that the same properties can be obtained throughout the entire composite structure [26]. Figure 2 presents a representative pressure-less infiltration apparatus.

Variables Affecting Pressure-Less Infiltration

Production of ceramic-metal composites through pressure-less infiltration is only possible when certain special conditions are met. The general variables important for spontaneous infiltration to occur are summarized in Figure 3 [27].

Surface Chemistry of Starting Materials (Surface oxide layer, coating of the surface with another phase, etc.):

The infiltration process is a highly sensitive proceeding process, and various factors with a wetting-reducing effect are present. Generally, oxide layers present on the molten surface prevent molten metal/alloys from wetting the ceramic material. These oxide layers create significant resistance to the infiltration of molten metal into the porous ceramic body. For example, aluminum metal has a very high affinity for oxygen, and at 400°C over 4 hours an aluminum alloy surface forms a layer 50 nm thick, and at room temperature instantly a layer 20 nm thick. Consequently, in Al-based systems, it is very difficult to eliminate oxide formation. In systems using Al alloys, infiltration is generally not observed below 900°C [31].

Infiltration Temperature:

Temperature is one of the most important production variables in the infiltration method, and at increasing infiltration temperatures, the fluidity of the liquid metal increases, making infiltration through the pores between ceramic grains easier. Rodriques-Reyes [24] reports that temperature is the most important variable in the production of Al-Si-Mg/SiCp composite structures using the pressure-less infiltration method. The infiltration of SiC block pieces was carried out with increasing temperature and decreasing dwell time, and it was noted that this process is particularly dependent on temperature [24]. Chong et al. [32] determined that when producing Al/SiCp composite structures using the pressure infiltration method, the threshold pressure decreased with increasing temperature. Additionally, it was determined that the pore amount in the composite structures decreased with increasing temperature [32]. Contreras et al. [33] produced Mg/TiC composite structures with 56% reinforcement volume fraction at 850-900°C in an Ar gas atmosphere using the pressure-less infiltration method. They observed that at increasing infiltration temperatures, the liquid matrix showed a successful wetting tendency toward the reinforcement. They noted that the mechanical properties of composite structures are largely dependent on wetting behavior and that mechanical properties are better at increasing infiltration temperatures [33]. Infiltration Time:For infiltration to occur, while depending on other process variables, a dwell time is required [24, 34]. Rodriques-Reyes [24] produced Al-Si-Mg/SiCp composite structures using the pressure-less infiltration method and determined that dwell time decreased at increasing infiltration temperatures. They noted that dwell time is a period dependent on temperature, but the combined effect of temperature and time variables could be disregarded [24]. Kevorkijan [35] infiltrated 356-T6 Al alloy containing 7% Si and 0.3% Mg into SiC, Si3N4, AlN, Mg3N2 and TiO2 ceramics using the liquid infiltration method. As a result, he determined that the infiltration distance increased linearly as a function of infiltration time and temperature [35].

Alloy Composition:

The compatibility between the reinforcement and matrix material is one of the important variables affecting the properties of composite structures. In the infiltration method, the wetting ability of the liquid metal toward the reinforcement significantly affects the infiltration distance. In numerous studies where aluminum matrix composite materials were produced using the infiltration method, it is reported that the Mg content of the liquid matrix shows an infiltration-increasing effect. It is known that in pressure-less infiltration, the addition of Mg to the liquid matrix and a nitrogen gas atmosphere facilitate infiltration. Mg provides infiltration by improving the wetting of ceramic particles by liquid Al [36, 37]. Aghajanian et al. [38] produced aluminum matrix composites reinforced with Al2O3 and SiC using the pressure-less infiltration method. In this study, while depending on production variables, it was noted that a critical amount of Mg is required for infiltration to occur. This required Mg amount is in the range of 0.5-1%. A temperature of 900°C, a duration of 5 hours, 100% N2(g) atmosphere and Mg addition are reported as optimum working variables [38]. It is reported that the addition of Cu to the liquid matrix delays infiltration, while the addition of Si increases infiltration [38]. In a study conducted by Chong et al. [32], when 2014 Al alloy was infiltrated into SiC bodies using the pressure infiltration method, the addition of 4.2% Cu to pure Al was reported to increase the threshold pressure [32].

Infiltration Atmosphere:

In some studies carried out to increase infiltration, it is reported that the process was carried out under Ar gas atmosphere up to infiltration temperatures or up to 1000-1150°C, then switched to N2 gas atmosphere at this temperature, and continued in N2 gas atmosphere during cooling. It is reported that this furnace gas atmosphere change facilitates infiltration [36, 39, 40]. Aghajanian et al. [38] produced aluminum matrix composite material with Al2O3 and SiC particle reinforcement using the pressure-less infiltration method. In this study, complete infiltration was obtained in a 75% Ar and 25% N2 gas atmosphere [38]. As the nitrogen content in the atmosphere increased, the AlN content also increased, and as the AlN content increased, infiltration increased [38].

Particle Size and Distribution of Starting Materials:

The voids created before infiltration between ceramic grains affect infiltration depending on ceramic grain size. Between ceramic grains with large grain size, the large voids before infiltration make it easier for liquid metal to infiltrate into these pores. Chong et al. [32] determined that increasing grain size reduced the threshold pressure required for infiltration when infiltrating 2014 Al alloy into SiC porous bodies using the pressure infiltration method. Elwahed and Asar [41] produced composite materials in the Al-Al2O3 system using the pressure infiltration method. In this study, they reported that infiltration distance increased with increasing reinforcement size and that the percentage of open pores decreased [41]. Candan et al. [42] infiltrated liquid Al alloy into porous SiC bodies with different grain sizes (12.8 μm-22.8 μm and 36.7 μm) at 400-900 kPa pressure and observed infiltration behavior with computer assistance. In this study, they reported that with increasing SiC grain size, infiltration time decreased [42].

Ceramic Volume Fraction:

In the infiltration method, ceramic volume fraction is one of the most important variables determining infiltration behavior and the properties of composite structures. Properties of composite structures such as porosity, hardness, tensile strength and wear are directly related to ceramic volume fraction. Additionally, reinforcement volume fraction has a significant effect in composite structure production by infiltration. For the production of composite structures using the pressure-less infiltration method, the ceramic volume fraction of prepared pellets generally varies between 40% and 70%. Important factors to be considered when determining ceramic volume fraction are as follows [43]: • The porous ceramic body prepared for the infiltration stage must have the required mechanical strength, • The porous ceramic body prepared to enable liquid metal infiltration must have sufficient permeability.

Pore Content of the Green Body:

In a study conducted by Glass and Green [44], Y-TZP samples were sintered at temperature ranges varying between 960-1305°C for 1 hour to produce bodies with densities varying between 56% and 88%. Molten nitrate salts were infiltrated into the prepared porous ceramic bodies. In this study, it was determined that infiltration depth is a function of infiltration time and initial body density. A linear relationship was found between infiltration depth and the square root of time. As pellet density increased, infiltration depth was observed to decrease [44]. Assoc. Prof. Dr. Ayşe Kalemtaş / Department of Metallurgy and Materials Engineering - Faculty of Natural Sciences, Architecture and Engineering Bursa Technical University

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