The Effect of Temperature on the Generation of Thermoinstability During Braking of Friction Lining Materials
Authors: Talib R. J. , Muchtar A. , Azhari C. H. 
Published: January 2013

Tenaga kinetik yang terhasil semasa proses pembrekan ditukar kepada tenaga haba dan bahan pelapik brek dan pad brek menyerap tenaga haba sebelum dilepaskan ke atmosfera. Tenaga haba yang terkumpul boleh menyebabkan bahan pelapik geseran automotif mengalami suhu yang tinggi dan memberi kesan kepada prestasi pembrekan. Dalam kajian ini, proses pembrekan disimulasi dengan menekan sepasang pad brek ke atas piring brek besi tuang kelabu pearlitik yang berpusing pada kelajuan pusingan malar 750 pusing seminit. Sampel dikenakan pembrekan berterusan dan berkala. Setiap sampel semasa pembrekan berterusan dikenakan lima masa pembrekan iaitu 3, 6, 9, 12, dan 15 minit dan beban kenaan 100, 200, 400, 600 dan 800 N. Manakala, setiap sampel semasa pembrekan berkala pula dikenakan kepada beban kenaan 600 N dan empat masa pembrekan berlainan iaitu 400, 800, 1200 dan 1600 saat. Seterusnya setiap sampel diperhatikan dengan menggunakan Kemikroskopan Elektron Imbasan (KEI). Pemeriksaan mikrostruktur menunjukkan ketidakstabilan bermula dengan pembetulan bebutir haba pada kawasan sentuhan hasil daripada suhu yang tinggi terjana semasa pembrekan. Haba terkumpul pada kawasan sentuhan menghasilkan tegasan haba yang mana akan menindih ke atas tegasan mekanikal, menyebabkan penjanaan mikroretak haba. Berdasarkan kepada keputusan mikrostruktur, dicerapkan bahawa mikroretak merambat, membesar dan akhirnya bertaut antara satu sama lain membentuk mikroretak berbilang apabila masa pembrekan ditingkatkan. Seterusnya, zarah haus tersingkir dari permukaan haus setelah mencapai jarak kritikal.

Kata kunci: Pad brek, ketidakstabilan haba, bebutir haba, micricetak haba, haus

In braking process, kinetic energy is converted to heat energy and the friction lining materials and brake pads absorb this heat energy before being released to the atmosphere. This accumulated heat energy may cause the automotive friction lining materials to experience high temperatures and this may affect the braking performance. In this study, the braking process was simulated by pressing a pair of brake pads against a rotating pearlitic gray cast iron brake disc at a constant rotating speed of 750 rpm. The test samples were subjected to continuous and intermittent brakings. In continuous braking, each sample was subjected to five different braking times of 3, 6, 9, 12, and 15 minutes and applied loads of 100, 200, 400, 600 and 800 N, whereas, in intermittent braking each sample was subjected to an applied load of 600 N and four different braking times of 400, 800, 1200 and 1600 seconds. Each sample was then observed under the Scanning Electron Microscope (SEM). Microstructural examinations showed that thermoinstability starts with the formation of thermogrannules at the contact area as a result of high temperature generated during braking. Accumulated heat on the contact area introduced thermal stresses which will then superimpose onto the mechanical stress, resulting in the generation of thermomicrocracks. Based on the microstructural results, it was postulated that microcrack propagated, grew and finally joined together to form multiple microcracks as the braking time increased. Subsequently, upon reaching a critical length, the wear particles are disposed from the wear surface.

Key words: Brake pads, thermoinstability, thermogranules, thermomicrocrack, wear