L.R. Boccaccini et al. Materials Characterization 54(2005)75-8 low resistance to thermal shock and their low ical loads resulting in microstructural damage. Meas- mechanical strength, which makes them unsuitable urements of the thermal expansion coefficient were as structural materials. The introduction of reinforce- utilised for the first time to assess the sensitivity of ments in the form of fibres in glass matrices, forming this parameter to microstructural changes, as a composites, can change these characteristics of possible method for detection of microstructural glasses making it possible to develop materials damage in its early stages of developI tolerant to microstructural defects or cracks. with evaluation of the mechanical behaviour of the high mechanical resistance and a very good resistance composite material after thermomechanical loads to thermal shock [1-4]. The composite materials thus was carried out through impact tests. Scanning obtained constitute advanced lightweight structural electron microscopy (SEM) was used to verify the materials, which can be utilised at high temperatures occurrence of energy-dissipation mechanisms, such as and under mechanical loads in oxidant atmospheres fibre pull-out, during composite fracture Therefore, these materials may find applications in energy-conversion systems, aerospace vehicles and structures, as well as in the building industry 2. Experimental fireproof components or thermal protection elements that must work under mechanical stresses [3-5]. This 2. 1. Material favourable flaw-tolerant behaviour of the composites is not only due to the high modulus of elasticity and The material investigated was a commercially mechanical resistance of the fibres but also due to th available unidirectional SiC-Nicalon&(NL 202) presence of a thin carbon-rich fibre/matrix interfacial fibre-reinforced borosilicate (duraN)glass-matrix layer(20-30 nm thickness). This layer, produced composite fabricated by Schott-Glaswerke (Mainz, during the high-temperature fabrication step, origi- Germany)[14. The composite was prepared by the nates energy-dissipation mechanisms during fracture, ethod. Details of this technique can be such as crack deflection and fibre"pull-out, provid read in previous publications [15, 16]. Nominal ing the appropriate matrix/fibre bonding strength properties of the matrix, fibre and composite are leading to favourable"pseudo-ductile"fracture behav iven in Table 1 [17, 18]. The material was received in iour [3, 5]. To achieve the satisfactory use of these the form of prismatic test bars of nominal dimensions materials in the applications above mentioned and to 4.5X3.8X100 mm. The density of the composites was extend their application to other technical areas, it is 2.4 g cm and the fibre volume fraction.4. This necessary to have a broad knowledge of their material is being considered for applications in the thermomechanical behaviour. This explains the grow- glass and non-ferrous metallurgy industries, for the ing quantity of studies published in the specialised fabrication of tools for handling of hot glassware and literature in the last decade about the behaviour of metal parts, respectively [14] glass-matrix composite materials when submitted to thermal gradients and combined thermomechanical stresses [6-13]. There is, however, further need in Table developing techniques to characterise microstructural Properties of composite constituents and of the composite [15-17] damage in composites after thermomechanical dam Property latrix DURAN Fibre sic age loads is required to assess the expecte (borosilicate Nicalon(40 voL% lifetime of components in service. The objective of (NL202) fibre content) this study is to investigate the influence of mechanical Density (g cm") 2.23 and thermal loads on the development of micro- Young's structural damage and in the resulting thermomechan- modulus(GPa) ical behaviour of silicon carbide(Nicalon)fibre- 0.22 0.200.2 Thermal expansion 3. 25x10 3×10-63.10×10-6 reinforced glass matrix composite materials. Thermal oefficient(K-) shock, thermal cycling, thermal aging and mechanical Tensile strength 60 600700 re-stressing were studied as typical thermomechan (MPa)low resistance to thermal shock and their low mechanical strength, which makes them unsuitable as structural materials. The introduction of reinforce￾ments in the form of fibres in glass matrices, forming composites, can change these characteristics of glasses making it possible to develop materials tolerant to microstructural defects or cracks, with high mechanical resistance and a very good resistance to thermal shock [1–4]. The composite materials thus obtained constitute advanced lightweight structural materials, which can be utilised at high temperatures and under mechanical loads in oxidant atmospheres. Therefore, these materials may find applications in energy-conversion systems, aerospace vehicles and structures, as well as in the building industry as fireproof components or thermal protection elements that must work under mechanical stresses [3–5]. This favourable flaw-tolerant behaviour of the composites is not only due to the high modulus of elasticity and mechanical resistance of the fibres, but also due to the presence of a thin carbon-rich fibre/matrix interfacial layer (20–30 nm thickness). This layer, produced during the high-temperature fabrication step, origi￾nates energy-dissipation mechanisms during fracture, such as crack deflection and fibre bpull-outQ, provid￾ing the appropriate matrix/fibre bonding strength leading to favourable bpseudo-ductileQ fracture behav￾iour [3,5]. To achieve the satisfactory use of these materials in the applications above mentioned and to extend their application to other technical areas, it is necessary to have a broad knowledge of their thermomechanical behaviour. This explains the grow￾ing quantity of studies published in the specialised literature in the last decade about the behaviour of glass-matrix composite materials when submitted to thermal gradients and combined thermomechanical stresses [6–13]. There is, however, further need in developing techniques to characterise microstructural damage in composites after thermomechanical dam￾age loads. This is required to assess the expected lifetime of components in service. The objective of this study is to investigate the influence of mechanical and thermal loads on the development of micro￾structural damage and in the resulting thermomechan￾ical behaviour of silicon carbide (NicalonR) fibre￾reinforced glass matrix composite materials. Thermal shock, thermal cycling, thermal aging and mechanical pre-stressing were studied as typical thermomechan￾ical loads resulting in microstructural damage. Meas￾urements of the thermal expansion coefficient were utilised for the first time to assess the sensitivity of this parameter to microstructural changes, as a possible method for detection of microstructural damage in its early stages of development. The evaluation of the mechanical behaviour of the composite material after thermomechanical loads was carried out through impact tests. Scanning electron microscopy (SEM) was used to verify the occurrence of energy-dissipation mechanisms, such as fibre pull-out, during composite fracture. 2. Experimental 2.1. Material The material investigated was a commercially available unidirectional SiC-NicalonR (NL 202) fibre-reinforced borosilicate (DURANR) glass–matrix composite fabricated by Schott-Glaswerke (Mainz, Germany) [14]. The composite was prepared by the sol–gel slurry method. Details of this technique can be read in previous publications [15,16]. Nominal properties of the matrix, fibre and composite are given in Table 1 [17,18]. The material was received in the form of prismatic test bars of nominal dimensions 4.5
3.8
100 mm. The density of the composites was 2.4 g cm3 and the fibre volume fraction ~0.4. This material is being considered for applications in the glass and non-ferrous metallurgy industries, for the fabrication of tools for handling of hot glassware and metal parts, respectively [14]. Table 1 Properties of composite constituents and of the composite [15–17] Property Matrix DURANR (borosilicate glass) Fibre SiC NicalonR (NL202) Composite (40 vol.% fibre content) Density (g cm3 ) 2.23 2.55 2.4 Young’s modulus (GPa) 63 198 119 Poisson’s ratio 0.22 0.20 0.21 Thermal expansion coefficient (K1 ) 3.25
106 3
106 3.10
106 Tensile strength (MPa) 60 2750 600–700 76 A.R. Boccaccini et al. / Materials Characterization 54 (2005) 75–83