Lina Jia

Third-body layer formation and local damage influence frictional interface evolution during cyclic braking. Clarifying the relationship between layer integrity and frictional response is therefore important for assessing interfacial damage in SiCp/Al braking pairs. An Al-10Si-3Cu-2Ni-1Mg composite reinforced with 20 wt.% SiCp was paired with a low-metallic friction material and subjected to cyclic braking at different pressures and speeds, with interrupted tests used to track early third-body layer development. Interrupted observations revealed Al₃CuNi fragmentation, SiCp/Al interfacial separation, and local material removal accompanied by debris generation and retention. Counterpart material transfer, together with debris mixing and compaction, promoted the transition from localized direct contact to third-body coverage. At 0.50 MPa and 80 km/h, relatively continuous coverage was observed after approximately 100 cycles, whereas the macroscopic frictional response stabilized only after approximately 400 cycles, indicating that coverage establishment alone did not mark the completion of running-in. At the same initial braking speed, increasing pressure from 0.50 to 1.00 MPa shortened running-in to approximately 150 cycles and intensified third-body layer cracking and local delamination, although steady-state friction coefficients differed by less than 0.02. The third-body layer exhibited insufficient coverage at low speed, more extensive coverage at intermediate speed, and pronounced local damage at high speed. These observations support third-body evolution involving formation, local damage, and material replenishment. Similar steady-state friction coefficients can coexist with different degrees of third-body layer damage. Interfacial assessment should therefore combine frictional response with layer integrity.

For SiCₚ/Al composites considered as lightweight brake-disc materials, increasing matrix strength may not improve wear resistance if brittle intermetallic phases promote cracking and near-surface material removal. This study therefore tests the hypothesis that Cu-induced changes in intermetallic morphology and spatial continuity can suppress damage and improve wear resistance in 20 wt.% SiCₚ/Al-10Si-xCu-2Ni-1Mg composites (x = 0, 2.5, 4, 5.5, and 7 wt.%) under dry reciprocating sliding against a Si₃N₄ ball. With increasing Cu content, the dominant Ni-rich intermetallics evolved from ε-Al₃Ni to δ-Al₃CuNi and subsequently to (δ-Al₃CuNi + γ-Al₇Cu₄Ni), while their morphology changed from discrete elongated/lamellar structures to short-rod or blocky forms and finally to a semi-continuous (δ + γ) network at 5.5 wt.% Cu. At 7 wt.% Cu, the (δ + γ) network became finer, but coarse Q-Al₅Cu₂Mg₈Si₆ and θ-Al₂Cu phases became more pronounced. In 5.5Cu, the semi-continuous (δ + γ) network and higher local matrix deformation resistance were associated with reduced interfacial damage, limited near-surface crack linkage, and a relatively compact mechanically mixed layer (MML). These features were accompanied by the lowest specific wear rate of (5.53 ± 0.02) × 10⁻⁴ mm³/(N·m) and a relatively stable frictional response over the 10 h test. At 7 wt.% Cu, fragmentation of coarse Q and θ phases was accompanied by pronounced ploughing, MML cracking, and delamination, while the specific wear rate increased to (10.59 ± 0.05) × 10⁻⁴ mm³/(N·m). These observations suggest that a semi-continuous intermetallic network can contribute to improved wear resistance through reduced damage and retention of a compact MML, but this benefit can be offset by fragmentation of coarse brittle phases and a less deformation-resistant matrix.

The performance degradation of composites caused by thermal cycling during braking is the main bottleneck limiting the application of aluminum-based composites in brake materials. In this study, through the design of dual-scale SiCp, induced thermal mismatch stress compensates for the decrease in hardness and wear resistance by thermal cycling, and the performance degradation under thermal cycles was effectively addressed. The addition of 1 wt.% sub-micron SiCp synchronously increase the elongation, tensile strength, yield strength and hardness by 29.8%, 15.3%, 20.3% and 12.5%. The main strengthening mechanism can be attributed to the noticeable coefficient of thermal expansion mismatch strengthening provided by sub-micron SiCp. A recrystallization model of composites with sub-micron SiCp is proposed according to particle stimulated nucleation of recrystallizations. Sub-micron SiCp provide higher thermal mismatch stress, driving the occurrence of recrystallization, while the substructure evolution of composites without sub-micron SiCp is mainly due to recovery.

The Al–10Sr master alloy is used to modify the A359/20 wt% SiC composite, aiming to enhance its tensile properties and wear characteristics concurrently. This modification yields improvements surpassing those observed in pure A359 alloys, a phenomenon attributed to the preferential formation of Al–Si–Sr compounds adjacent to SiC particles. The results elucidate that the integration of SiC, serving as a wear‐resistant phase, substantially augments the wear resistance of the alloy. Abrasion marks incurred from singular friction events are discernibly more superficial on eutectic silicon in contrast to the aluminum substrate. Furthermore, the metamorphosis of eutectic silicon from an elongated to a granular form mitigates stress concentrations. This morphological alteration is pivotal, enhancing wear resistance and diminishing the likelihood of particle detachment.