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Cryogenic Treatment Disc Brake Seminar report
As part of a safety critical system in passenger vehicles, disc brakes and the materials they are made of must meet a number of requirements. They must have high thermal resilience, a stable coefficient of friction with temperature to avoid brake fade, provide a reproducible uniform response as well as wear uniformly during their service life. Clearly, functional reliability is crucial. Disc brakes represent a complex tribosystem, with the brake rotors forced into contact with brake pads that are typically composite materials with a number of additives to enhance their thermal characteristics, held together by a phenolic resin binder. The contact conditions between these components are extreme, with high clamping forces and temperatures in the friction ring capable of reaching 700 ◦C over long downhill stretches. As around 90% of the kinetic energy of the vehicle is absorbed by the brake rotors as heat, they must be able to rapidly dissipate energy to ambient air. During braking, the temperature rise is significantly determined by the kinetic energy and therefore the mass of the vehicle as well as the heat capacity and therefore the mass of the brake rotor. Intuitively, larger brake rotors result in a lower rise in temperature and less susceptibility to brake fade, where the coefficient of friction between the brake rotors and pads declines significantly with increasing temperature, leading to greater stopping distances. Brake material manufactures therefore have to optimize between the deformation and deflection of brake rotors under braking forces, weight and thermal performance, whilst providing for a long service life (105 km). For these reasons grey cast iron (GCI) has become the material of choice for passenger vehicles, with its castability and machinability being important factors in its widespread use.
The use of grey cast iron vs. its alternatives:
Pearlitic grey cast irons are the primary material used in brake rotors for passenger vehicles, usually with small quantities of alloying elements added to enhance desirable properties. The high graphite content of these materials improves thermal conductivity and the large heat storage capacity of GCI allows brake rotors to avoid overheating and prevent brake fade during use. Additives such as chromium and molybdenum give greater resistance to abrasion and improved heat cracking behaviour. The addition of manganese refines the pearlite matrix and improves hardenability whilst manganese and silicon are added as de-oxidants to improve corrosion resistance. Importantly, silicon acts as a graphite stabiliser and prevents the formation of iron carbides. As demonstrated by Hecht et al., graphite flake morphology has a significant effect on the thermal diffusivity of SAE G3000 (now G10) GCI. Using the flash technique they found that for GCI with the same chemical composition, a 50% increase in thermal diffusivity was related to a fourfold increase in the size of graphite flakes. The interlocking nature of graphite flakes in GCI enhances its thermal conductivity. The benefits of using GCIs are a high thermal storage capacity, thermal conductivity and resistance to brake fade coupled with a high degree of castability and machinability, lending to low manufacturing costs. The main drawbacks of these materials are the large and therefore heavy brake rotors needed, as well as poor corrosion resistance. In recent years for passenger vehicles and previously for niche and high-performance applications, other types of materials have been studied and used as replacements for GCI. Cueva et al. investigated the possibility of using compacted graphite iron (CGI) as a replacement for GCI. During comparative pin-on-disc testing it was shown that the same friction force, temperature and similar wear rates were found in CGI at lower contact pressures than were required for GCI. Other alternatives include metal matrix composites (MMCs), carbon–carbon (C/C) composites, ceramic matrix composites (CMCs) as well as titanium (Ti) alloys. The most popular MMCs suggested for replacing GCI in brake rotors are those based on aluminium, as Al- MMCs demonstrate similar tribological performance whilst offering significant weight savings. Unfortunately it suffers from a low melting temperature, limiting its applications. C/C composites are commonly used in motor racing and aircraft applications, where resistance to extreme temperatures and the lowest possible weight are demanded. Apart from the inherent cost of these materials, C/C composites show poor braking performance at low temperatures. CMCs are a more viable alternative, demonstrating significant weight savings over GCI as well as thermal stability up to 1300 ◦C. These are notably used in performance and luxury vehicles, as well as for aircraft brake systems, but are still prohibitively expensive for widespread use in passenger vehicles. CMCs such as carbon fibre reinforced ceramics with a matrix containing silicon carbide (C-SiC), possess the following benefits when compared to GCI: improved resistance to abrasion and service lives of up to 300,000 km, a two-thirds reduction in weight, high thermal stability and corrosion resistance. However, it is also prohibited from more widespread use due to the complex and costly manufacturing processes required to produce brake rotors. These drawbacks are common with all potential lightweight alternatives to GCI, meaning it is likely to remain the primary brake rotor material in passenger vehicles in the near future. With all composites, there are also environmental and health concerns regarding the production of fine particles during braking that have yet to be sufficiently addressed.
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