Advanced ceramics are an important class of engineering materials which offer numerous enhancements in performance, durability, reliability, hardness, high mechanical strength at high temperature, stiffness, low density,, electrical, and thermal insulation, radiation resistance, and so on. [1, 2]. Among various ceramics materials, silica exhibits a unique combination of properties such as high melting point with high fracture toughness, which makes it suitable for various technological applications [3]. However, direct application of monolithic silica for structural components is not appropriate due to its inferior flexural strength (20–30 MPa), lower compression strength (60 MPa) and extremely low fracture toughness (0.62 MPa m½) [4]. Thus, it is necessary to improve mechanical properties of monolithic silica to make it acceptable for certain structural applications.
One of the means of achieving advanced ceramic materials with improved mechanical properties is by using either particulate or networks of continuous fibers as reinforcements with silica as a matrix material. This leads to newer structural materials, known as fiber-reinforced ceramic–matrix composites [4, 5]. It is expected that a combination of glass fiber as a reinforcement and silica as a matrix has the potential to overcome the drawbacks of brittleness with enhanced damage tolerance.
Literature shows that silica matrix based composites can produce near net shape products with improved fracture toughness and impact resistance at minimal cost than monolithic silica [6]. However, in comparison to particulate based silica composites, continuous silica fiber reinforced silica matrix composites has gained more attention. Such composites can be developed at relatively higher temperatures through chemical vapor infiltration and hot slurry impregnation methods, however the risk of fiber degradation at high temperatures limits the effectiveness and utilization of such methods. In addition, high temperature treatment in oxidizing environments introduces a limitation to thermo-mechanical and thermo-chemical compatibilities [7–9]. On the other hand, a relatively newer processing method to fabricate continuous fiber reinforced silica matrix composites in the form of sol infiltration (SI) technique has emerged as a promising alternative to high temperature processing routes.
The SI method involves low densification temperatures with minimal shrinkage and reduced drying stresses. The utility of SI method as an effective silica matrix composites fabrication technique has been presented in recent studies conducted by numerous researchers [5, 10, 11]. These researchers have incorporated continuous silica fibers with silica matrix through silica sol infiltration (SSI) technique to obtain a silica matrix composite with enhanced mechanical properties [4, 5, 12, 13]. Prasad et al. studied the elastic behavior of silica-silica fiber-reinforced ceramic matrix composites that were fabricated through silica sol infiltration sintering method (SIS) [14]. Similarly, Kim et al. also studied the mechanical properties of 2-D silica-silica continuous fiber reinforced ceramic matrix composites [15]. Also, Liu et al. incorporated 2.5D silica fibers into silica matrix through SIS process to achieve a toughening effect in composites [16]. Additionally, Li et al. prepared 3D seven directional braided silica composites through SIS approach [13]. In another study Liu et al. performed mechanical testing of 2.5D silica reinforced composites prepared through SIS approach. The achieved flexural strength for shallow bend joint was 50.3 MPa while for shallow straight joint the strength was 48.4 MPa [16]. These researchers concluded that the incorporation of continuous silica fibers into silica matrix provided the composites with a kind of pseudo ductility by preventing catastrophic crack growth by such mechanisms as fiber debonding, matrix cracking, fiber-pull out and bridging effect.
In the present study, a systematic yet relatively a fresh approach has been defined to develop glass fibers reinforced silica matrix (GFS) composites through SSI method. This method was selected due to its effectiveness in relatively low densification temperature, low shrinkage and reduced drying stresses. The approach involves multiple infiltrations of silica sol into fabric preform under vacuum followed by multiple drying cycles. The multiple infiltrations, aid in homogenizing the distribution of nano silica in the preform and vacuum assistance ensure maximum voids removal. To study the behavior of GFS composites, microscopic characterization of the fibrous preform was performed after 3rd, 5th, 7th and 9th infiltration cycles. The coating of nano silica on preform after selected infiltration cycles were investigated under a field emission scanning electron microscope. Afterwards, mechanical testing was performed on final sintered GFS composites to assess the strength and flexural behavior of GFS composites. Finally, the results were compared with the available data in terms of relative improvement in mechanical properties.