Preparation Of Phenolic Braking Materials And Their Friction And Wear Performances

The good quality brake materials should have reliability, safety, comfort and wearability pertaining to the modern automotive brake systems. It is a key to appropriate and stable friction coefficient. Brake material wear is a complexity project. The establishment of the wear equation has great theoretical significance and practical value. Light-weight brake rotors and brake drums are the hottest research subject in today's world. SiC particle-reinforced aluminum metal matrix composites (SiCP/Al-MMCs) are possible replacement materials for the conventional cast iron.Friction and wear properties of ingredients used in brake material are investigated by using a D-MS constant-velocity tester. It has been found that when the weight ratio between resin and steel wool fiber reaches 1:4 under investigation, the composite material possesses better friction, wear as well as processing and molding performances. Therefore, it is selected in the following work. Potassium titanate whisker exhibits better friction and wear performances as compared with barite. This correlates well to its stable chemical structure and whisker performance characteristics. Copper fiber is an excellent yet expensive ingredient of brake material. Black iron oxide is merely a mild abrasive at low temperature, whereas aluminum oxide is a strong abrasive at low and high temperatures. When the weight ratio of coke, graphite and antimony trisulfide is 5:3:3, its friction coefficient is optimal for the complementary effect. This could be well attributed to the complementary action of the low temperature lubricity of coke and graphite as well as the high temperature lubricity of antimony trisulfide. These findings provide important theory and experimental evidence for determining the following friction (overlay) material formula.By applying the above experimental results and orthogonal experimental design and after overall evaluation on Chase Machine test results relevant to temperature, load and velocity, the screened optimal semi-metallic disc overlay material formula consists of 10resin/40steel wool fiber/4copper powder/7potassium titanate whisker/10barite/2 aluminum oxide/5black iron oxide/10coke/6graphite/6antimony trisulfide (in mass%). Little has changed about friction coefficient with respect to temperature, load and velocity and its average friction coefficient is 0.444. Also, the wear is small. On the other hand, after appropriate raw materials choice, orthogonal experimental design and testing adhesive performances between underlayer material and steel plate by using a shear strength tester, the screened optimal semi-metallic disc underlayer material formula consists of 10.5resin/29.5steel wool fiber/7.4wollastonite/8.4coke/5.3black iron oxide/2.1flax/36.8rockwool (in mass%). Its shear strength is 4.7 N·mm-2 and 2.8 N·mm-2 at two different temperatures of 30℃and 200℃respectively, and its shear residual area is 99% at 30℃.A wear equation for brake material during automotive braking has been established. The derived wear and number of allowable brakings diagrams from the equation could conveniently be used to evaluate wear per braking and serve life for brake material. Based on the previous formulas, the finished products were manufactured, followed by the friction property tests on an inertia dynamometer. Results reveal that friction coefficient is very well included within the range of the required discrete band for Volkswagen Company, demonstrating that the formula is of great applied value. In order to further investigate the equation, the products were installed in Santana taxis for identical car types in three different cities. Vehicle tests were preformed. Results show the average weight loss per braking increases with average initial braking velocity, whereas the average nominal specific wear rate decreases firstly and then increases. These findings are consistent with the analytical predictions of the equation. Three average nominal specific wear rate values are fully contained within the permitted range of practical specific wear rate values, which further confirms the validity of the equation.Friction and wear performances between SiCP/Al-MMCs and the corresponding brake material are investigated. Two identical-size brake drums available for the Chase Machine test were made of aluminum matrix composites reinforced with two sizes of SiCP (3.5μm and 34μm,25 vol.%), respectively. The same brake materials were tested against each individual Al-MMCs drum on the Chase Machine. Results indicate the friction and wear performances observed for the brake material against the drum with large-size SiCP were better than those against the drum with small-size SiCP. The friction performances were found to strongly depend on the size of SiCP and the wear resistance on tribofilm thickness and SiCP pullout, confirming that Al-MMCs containing large-size SiCP are suitable for the manufacture of brake rotors and brake drums. Furthermore, the friction coefficient continuously decreased with the increase of load and speed, and converged gradually at two temperatures of 177℃and 316℃. Friction fade took place at high temperatures, followed by excellent recovery upon cooling. Also, the specific wear rate decreased with the increase of load and speed, but increased with temperature.In order to improve the above low friction levels, the initial 34μm SiCP content of 25 vol.% was increased to 30 vol.%, and four brake material formulations were determined by increasing ZrSiO4 rate as 0, 4, 8, and 12 mass% with the highest Mohs hardness among all the ingredients. The brake materials were measured on the Chase Machine with the Al-MMCs drum similar to the above. Results show friction was obviously ameliorated with the increase of ZrSiO4. The brake material had relatively good friction performances and the best wear resistance when the percentage of ZrSiO4 amounted to 8. Friction decreased for the applied load range from 669 to1069 N, but increased from 1069 to 1469 N at 316℃when to 12. Though friction reaches the best level, the risk of excessive wear is enhanced. The presented friction models can explain the friction and wear properties intimately associated with the content of ZrSiO4 on the worn surface of the brake material and in the tribofilms better.