When the load was increased from 20 N to 30 N in case of C/C normal, wear debris got pulverized and compacted, and formed friction film on the surface of composite which led to decrease in COF. However, when the load was increased beyond 30 N, COF increased. This can be attributed to the accelerated formation and disruption rate of friction film at higher loads. Disruption rate was more than formation rate. Simultaneous formation and disruption of friction film can be observed in Fig. 14a.
As the sliding velocity was increased, COF of C/C normal first increased up to 2 m/s sliding velocity. After that, it decreased for 2.5 m/s and again increased for 3 m/s. The first rise in COF was due to increase in breaking energy which led to rapid desorption of water vapour and oxygen from the contact surface [10, 25]. When the sliding velocity was increased beyond 2 m/s, further increase in braking energy led to increase in temperature due to which pulverization and compaction of wear debris was easy and continuous friction film was formed on the surface. This decreased the COF at 2.5 m/s. Again increasing the sliding velocity led to accelerated ejection of wear debris due to centrifugal force  which again increased the COF. Figure 14b shows C/C normal tested at 3 m/s sliding velocity. Some broken fibres (in the form of small fragments) and carbon debris can be observed. However very less friction film was formed.
In case of C/C parallel, the increase in COF up to 30 N load was due to deeper penetration asperities with increased load which increased resistance to sliding. However as the load was increased further, formation of friction film led to decrease in COF. Further increase in load led to rapid formation and disruption of friction film which led to increase in COF. Discontinuous friction film at 60 N load can be observed from Fig. 15a.
Formation of friction film at low sliding velocity and its accelerated rate of formation and disruption due to spreading of wear debris at higher sliding velocities  describes the nature of COF with increase in sliding velocity. Figure 15b shows C/C parallel at 2.5 m/s velocity and 20 N load. It can be observed that at high sliding velocity, wear debris didn’t pile up much on the surface. Some broken fibres were observed.
C/C normal showed opposite friction behaviour as compared to C/C parallel. In case of C/C parallel, the formation of smooth surface was easy due to more surface porosity and filling up of pores by the generated wear debris. However in case of normal orientation of laminates, formation and disruption of friction film effected the friction film when load and sliding velocity were varied.
COF of C/C-SiC normal and C/C-SiC parallel increased as the load was increased. This was attributed to the deeper penetration of hard SiC and second phase Si particles into the counterface as the load was increased [6, 19, 26]. There is also free silicon in case of C/C –SiC composites which plasticizes at higher load and led to adhesion at higher loads, thereby increased COF . Figure 16a shows C/C-SiC normal tested at 30 N load and 2 m/s sliding velocity. SiC particles in wear debris were observed.
C/C-SiC normal showed decrease in COF when the sliding velocity was increased. As sliding velocity was increased, the formed wear debris spread more easily on the surface. Debris contained hard SiC and second phase Si particles. SiC and second phase Si particles can’t be cut easily. The spread debris rolled in between the contact surfaces. Figure 17a shows C/C-SiC normal tested at 3 m/s sliding velocity and 20 N load. Wear debris can be observed.
COF of C/C-SiC parallel increased when the sliding velocity was increased. This was due to resistance provided by breaking of fibres . As the sliding velocity was increased, the particles which were ejected from the surface slid from the surface due to centrifugal force  and fibres directly came in contact with the counterface. The increase in braking energy due to increase in sliding velocity led to breakage of fibres. More fibres broke at higher velocities which increased COF. Broken fibres can be observed in Fig. 17b.
C/C-SiC parallel generally showed more COF as compared to C/C-SiC normal. COF of composites depend on the interaction and local COF of different constituents . C/C-SiC parallel contained more proportion of SiC as compared to C/C-SiC normal. Thus, C/C-SiC exhibited higher COF as compared to C/C-SiC normal.
COF of C/C disk first increased and then decreased a bit when load was increased in case of non-conformal Hertzian contacts. This was attributed to the easy formation of friction film due to localized stress regions. However for C/C-SiC disk, formation of friction film took place at moderate loads. But at high loads, SiC disrupted the formed friction film and abraded the steel ball which increased COF at high loads. The difference between COF values from low conformity contacts and non-conformal Hertzian contacts was large in case of C/C composites as compared to C/C-SiC composites. The COF value was less in case of non-conformal Hertzian contacts as compared to low conformity contacts because the contact area in case non-conformal contacts was very much petite and generation of high and localized stresses led to easy formation of friction film which decreased COF.
Wear loss of C/C and C/C-SiC composites increased with increase in load whether it was normal or parallel orientation of laminates. It was observed that the composites with parallel orientation of laminates showed less wear loss as compared to composites with normal orientation of laminates. This was due to more surface porosity in case of parallel orientation of laminates. Wear debris filled the pores and formed a smooth surface. However at higher sliding velocities, wear loss of composites having parallel orientation of laminates was more. This was because of easy spreading of wear debris at high sliding velocities due to which smooth surface formation didn’t take place.
Wear loss of C/C composites was more as compared to C/C-SiC composites in non-conformal Hertzian contacts. Figure 18a shows C/C disk tested at 40 N load and 2 m/s sliding velocity in non-conformal Hertzian contacts. It was observed that due to formation of grooves in the vicinity of contact area, wear debris didn’t get escaped much. Broken fibres can also be observed. Thus at higher load, some wear debris pulverized, and the un-pulverized particles acted as third body which rolled in between the contact surfaces and decreased the wear loss at higher loads. In case of C/C-SiC composites, the wear debris contained hard SiC particles as can be observed in Fig. 18b.
SiC particles are hard to cut and pulverize even at high loads. Thus wear loss of C/C-SiC composites was lower than C/C composites in non-conformal Hertzian contacts.
Wear loss in case of non-conformal Hertzian contacts was more as compared to low conformity contacts due to generation of high and localized stress regions. The pressure corresponding to the same load (as in low conformal contacts) was very much high which led to increased wear loss.