PSI - Issue 33

Pietro Tonolini et al. / Procedia Structural Integrity 33 (2021) 1152–1161 / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction The braking system is an essential system for the correct functioning of a vehicle, both from a technical point of view and for the safety of passengers and other road users. Nowadays, the most common solutions still involve the use of drum brake or disc brake systems or a combination of these. In the case of a disc brake system, during braking, the pistons inside the caliper assembly press the pads against the two brake bands of the disc, converting part of the kinetic energy of the vehicle into heat, generated by the friction between the coupled materials (Andrews, 2014 ; Gelfi, 2016 ). A disc brake consists of two main parts: the brake bend and the bell, which is the connecting element between the wheel hub and the disc. In order to guarantee the needed mechanical performances combined with passengers comfort, the brake bend material must have good conductivity and thermal diffusivity, a good damping capacity together with adequate mechanical, thermal and corrosion resistance (Maluf et al., 2004). In the European market, lamellar gray cast iron (GCI) with a pearlitic matrix is still the most used material for the production of braking bands that, despite its high density and low corrosion resistance (Djafri et al., 2014), is characterized for most of the aforementioned properties together with a good castability, workability and low cost (Grabiec, 2014). However, braking systems are also responsible for the direct emission of airborne particulate matter in the environment as a result of both pads and disc wear, especially in the urban areas, contributing for 55% of the total non-exhaust traffic related PM10 emission and for 21% for PM2.5 (Grigoratos and Martini, 2014). Despite the absence of legislative limits, it is however imperative limiting the production of these particles which are harmful to human health (Gonet and Maher, 2019). Recent academic studies have already shown the possibility of reducing these emissions by increasing the wear resistance of the disc through heat treatments (Perricone et al., 2018) or with the use of wear resistant coatings (Aranke et al., 2019). For example, high-velocity oxygen fuel (HVOF) technique has been employed for the deposition of Cr3C2-NiCr and WC-CoCr hardmetal coatings (Federici et al., 2017; Wahlström et al., 2017; Menapace et al., 2019), resulting effective in the enhancement of the disc wear resistance together with a reduction in the emission performance. In fact, these solutions are already available in the market for premium passenger cars (Elbrigmann, 2017; Bosch, 2020). Laser cladding is another deposition technique suitable for overlay welding onto GCI braking bends (Aranke et al., 2019; Gramstat et al., 2019; Dizdar et al., 2020). Laser cladded composed by a Ni-self fluxing alloy + 60% spheroidized fused WC has demonstrated trough pin on disc (PoD) tests higher wear resistance and lower particles number concentration expelled with respect to the reference GCI samples (Dizdar et al., 2020). However, some harmful airborne particles are still emitted into the environment. It is therefore necessary to limit the use of chemical elements that are harmful or with suspicion of damage to human health more than Fe particles in the disc brake system components subject to wear. In fact, elements like Ni and Co, that are widely used as metal matrix substances in coatings containing cemented carbides have been classified as serious health hazard material by the European Chemical Agency (ECHA) due to the recognition and concerns on skin and respiratory sensitizing, while for W - substances the classification are less serious (Echa, 2020). Stainless steel powder alloy is a possible candidate for the replacement of Ni and Co metal matrix in wear and corrosion resistant laser cladded coatings (Vilhena et al., 2016; Zhang and Kovacevic, 2019). Recent work regarding the deposition of a martensitic stainless steel powder on disc brake rotor has demonstrated through PoD tests against a commercial low metallic, Cu containing, pad material a higher mass loss and a higher PM emission rate than the conventional GCI rotors. However, no carbides were added in the powder mix. In this regard, this work is focused on the possibility to replace wear resistant coating hazardous materials for GCI disc brake by stainless steel powder deposited via laser cladding. In particular, two powders were deposited: a stainless steel powder and the same powder reinforced by the addiction on tungsten carbides (WC) particles. The wear behavior was evaluated simulating the sliding in operation through laboratory tests as PoD and block on ring (BoR) tests utilizing commercial pad materials as counterpart. For comparison, HVOF samples for PoD test was machined from a commercial coated brake disc while pearlitic GCI samples was realized for both disc and ring production.