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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

With a KIC

With a KIC of 9.5 MPa·m 1/2 , AOPC20IS showed the highest mean KIC combined with the lowest scatter of all MMCs. Marginally lower values were determined for AGPC15IS, where the preform was fabricated with a glassy binder. The TOPC10IS MMC and the hybrid- reinforced composite FATOIS showed comparatively lower mean KIC values between 7.2 and 7.4 MPa·m 1/2 combined with a larger scatter. The fracture surface of a tested sample of TOPC10IS was investigated using SEM. Figure 4.81 a) shows a typical optical micrograph of a similar MMC sample to represent the examined areas. These were the intergranular area with coarse metal ligaments (Figure 4.81 b) and the intragranular area with small ligaments between the single particles (Figure 4.81 c). b) a) 2 µm 181 c) 2 50 2 50 µm µm 2 µm Figure 4.81 Fractographic microstructure of TOPC10IS. a) Representation of the regions examined in an optical micrograph. SEM of the fracture surfaces: b) Intergranular and c) intragranular region.

Significant deformation developed in the intergranular area prior to fracture, as indicated by the fissured surface with deep cleavages. A platelet-shaped phase was observed on the apex of a deformed alloy volume as indicated by the arrows. The plane fracture surface of this phase indicates brittle fracture. The position of it in the alloy, its shape and the brittle behaviour indicated that Si in the alloy acted as the focus for failure. As it was free of residual Al, low adhesion of the surrounding α-Al on the Si-grain is indicated. The surfaces of the ceramic particles in the intragranular infiltration region were free of adhering alloy, Figure 4.81 c). The metal ligaments between the particles were fissured and are marked with an arrow. This indicated ductile fracture of the metal alloy. In contrast, the fracture surfaces of the ceramic particles were plane and therefore brittle fracture occurred but fracture was dominated by intergranular fracture across the grains. 4.10.4 Tribological properties Tribological testing was carried out by sliding a steel cylinder on a disc of a fine polished surface of the MMC in reciprocative sliding contact with oil as a lubricating medium. The resulting time-friction coefficient curves were analysed as described in 3.5.6 to give the friction coefficient f50m. After the test, the width of the sliding path was evaluated as a measure of the wear. The f50m values and the width of the wear path ww are presented in Figure 4.82 for the pure alloy IS and the MMCs. The maximum scatter in f50m of three test runs was ±0.02. The lowest value of f50m of 0.15 was measured for the AODY30IS. The MOPC20IS, AOPC20IS and AG15IS MMCs had intermediate f50m values between 0.19 and 0.20. The pure alloy IS and the TOPC10IS MMC had similar friction coefficients of 0.24. The highest friction coefficient of 0.26 was found for FATOIS. Apart from AODY30IS, ww of the tested samples were between 250 µm and 670 µm as shown in Figure 4.82. The maximum wear (670 µm) was found for the pure alloy IS and the 182

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