Laser cladding is frequently applied for development of wear and corrosion protective coatings on grey cast iron brake rotors. For vehicles that use mostly regenerative braking corrosion protective function is most important to provide reliable friction coefficients despite long idleness of friction brakes. For vehicles with friction brakes only minimized particle emission is a key factor to fulfill current legislative requirements. To avoid release of harmful particles into the environment nickel, cobalt and copper free coatings are desired. Stainless steels are considered as cost effective materials and suitable laser cladding processes have been developed to produce such claddings on grey cast iron brake rotors at deposition rates exceeding 12 kg/h and surface coverage rates exceeding 10 m2/h. But without hard phase reinforcement only rather poor wear resistance is achieved. Among the hard phases that could be used for reinforcement of stainless steels spherical fused tungsten carbide has been applied frequently and permits production of claddings with up to 30 vol.-% carbide content. However, tungsten carbide easily dissolves in stainless steel melt forming brittle mixed carbides. Therefore, the applicable process parameter window is very narrow. NbC and TiC show much better metallurgical compatibility, because they show lower tendency to dissolve in stainless steel melts and they re-precipitate without formation of brittle phases. TiC reinforcement results in a little higher crack formation tendency compared to NbC but shows advantages concerning pricing and availability of raw material. Different kinds of TiC (composite) powders have been applied for reinforcement of stainless steel laser claddings on grey cast iron brake rotors. Pure TiC powders that are produced by sintering and crushing as well as plasma spheroidization result in reinforcement by coarse, i.e. 10-50 µm sized, carbide particles. For use of TiC/FeCr composite powders produced by sintering and crushing or spray drying and sintering carbide size is only about 1 µm. The different TiC (composite) powders are evaluated concerning their impact on feed rate stability, on dissolution and embedding in the stainless steel matrix and on strength of the produced claddings. The irregular shape of sintered and crushed TiC powders results in feed rate stability restrictions and relatively strong dissolution in the steel matrix melt. Cracks formed during the crushing procedure are not completely penetrated by steel matrix melt and represent relatively large crack initiation sites. Plasma spheroidization of TiC powder permits achieving high feed rate stability and avoiding presence of cracks in carbide particles. However, the spheroidization process still needs to be optimized to achieve full melting of TiC particles larger than 30 µm and increases powder costs significantly. Spray dried and sintered TiC/FeCr composite powder permits excellent feed rate stability and low dissolution in the steel matrix melt. However, large particles that are not penetrated by steel matrix melt show only low cohesive strength and can easily cause microcrater formation during grinding of claddings. Sintered and crushed TiC/FeCr composite powder shows good feed rate stability, low dissolution in the steel matrix melt and high cohesive strength within composite particles.
Dr.-Ing. Andreas Wank, Head of Research and Development, GTV Verschleiss-Schutz GmbH; Mr. Christian Schmengler, Research engineer, GTV Verschleiss-Schutz GmbH; Ms. Diana Pehl, Research engineer, GTV Verschleiss-Schutz GmbH; Ms. Annika Krause, Research engineer, GTV Verschleiss-Schutz GmbH; Mr. Sascha Barteck, Research engineer, GTV Verschleiss-Schutz GmbH