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In recent years, several new types of automatic transmissions have been developed and launched such as CVT, AMT, DCT and Multi Step AT. The operating condition of wet friction material in automatic transmission has become more severe in order to achieve further improvement of fuel economy and compact size with high power engine. Under higher speed engagement, the sliding interface between friction material and separator plate (steel) must absorb a large amount of energy in a very short time. Recently, there is increasing to focus on the hot spot phenomenon that is appeared as some discoloration and distortion on reaction plate for wet friction material. This hot spot phenomenon is known as a typical failure mode of wet clutch system, because it influences the durability and shift quality in clutch system. [1][2] Hot spots can cause degradation of friction performance during shift change in automatic transmission.