Towards green braking

US regulations are driving the rest of the world towards a definitive reduction of copper in brake-pad compounds, mainly for environmental reasons. Indeed, we can ultimately foresee copper being entirely eliminated from friction materials as a direct result of US environmental restrictions. Within the brake-friction-material industry, there is a clear push towards the use of “global” materials for economy-of-scale reasons. It is self-evident that manufacturers of brake linings would prefer to sell the same family of materials worldwide, but at the same time OEMs do not want to risk jeopardising their braking performance in any strategic market.

Copper is used in brake compounds in order to improve brake effectiveness, but also for the efficient transfer of heat. It is extremely difficult to substitute this component without causing any undesirable side effects. The industry is currently using mixture of different components in order to reduce copper content, but this is likely to have an impact on quality.

In terms of brake effectiveness, at present cu-free materials show quite a good nominal coefficient of friction, but produce a lower recovery capacity in comparison with traditional “low-met” materials, especially when it comes to NAO (non-asbestos organic) cu-free material. High-speed, high-temperature and high-load performance is clearly an issue for NAO cu-free material, which leads us to believe that while the future in Europe could be copper free, it is unlikely to be NAO copper free owing to the very high performance requirements.

Another critical aspect to consider is corrosion. Applus+ has been working to develop an internal methodology to evaluate the effects of corrosion and the propensity for corrosion removal (especially for NAO materials), making full use of our brake dynamometers, located in Nuneaton, UK. This methodology allows us to evaluate BTV (brake torque variation) together with pressure variation and acceleration at critical points in a brake system. The successful development of a dedicated dynamometer control and logging system, based on the same architecture used for brake NVH and performance testing, represents a key milestone. We recognise the importance of brake-dynamometer testing in the complete vehicle-development process, as it is a powerful tool to help meet tight schedules for friction-material selection or brake-NVH tuning activities in the early stages of development.

NVH is another key element to consider, with “Squeal, Judder and Groan” set to become hot topics for cu-free development in the very near future. For this reason, we have deepened our pure NVH knowledge through close cooperation with our NVH Department, which provides experts to work alongside our brake team on a daily basis. The relationship between the areas of braking and noise & vibration has become ever closer over the last four years, which has allowed us to develop specific tools for NVH problem solving. We can now measure the source of brake noise or vibrations extremely close to the source (the friction material). We have, for instance, developed the ETPA (Extended Transfer Path Analysis) methodology: a tool that enables us to investigate how vibrations are transmitted from the friction pair up to the driver. Many related activities have been carried out in order to validate this tool.

When it comes to “global friction materials”, we also need to provide a global, integrated support service to our customers right from the initial concept phase through to post-SoP achievement. As such, we have strived to integrate a range of functions under the same umbrella. Our teams of design, simulation and testing experts work closely together so as to share their variety of approaches and knowledge. This integrated approach has proved effective in many recent projects, where we have successfully implemented a “function-led design” philosophy. Our extensive experience of turnkey projects in the Braking System Department has convinced us of the value of this kind of integrated approach when dealing with global materials, as cu-free materials are intended to be.

Brake NVH is of vital significance to the areas of electric vehicles and regenerative braking, where the concept of silent braking is rapidly gaining in importance. It is not acceptable, for example, for brakes to be noisier than powertrain. The tendency in more recently developed vehicles has been to move towards the use of regenerative braking, customising traditional hydraulic brakes to emergency applications. In principle, this could lead to a reduction in brake dimensions, even if this is not yet evident in the market. It goes without saying that lightweight brake design is the direction in which we are heading. We therefore need to work on integrating our regenerative-braking know-how with our expertise in NVH-related brake design.

And last but not least, we are also working on the other “hot topic” within our industry: brake-wear emissions. All of the major international OEMs and friction-material manufacturers are carrying out work in this area, which represents one of the brake industry’s biggest challenges in the short- and medium-term. We are involved in the main working groups as well as collaborating closely with our emissions colleagues in the Powertrain Department, which serves to illustrate that innovation is paramount in the IDIADA Division.

By Fabio Squadrani