High temperature tribological behavior of surface coated tool steel

The tribological characteristics are influenced by many factors and materials of contacting surfaces play very important role in tribology. By studying the tribological behavior of materials, we can make an appropriate choice of material pairs to meet the specific requirements of a specific application and save energy, conserve materials, and reduce weight and cost of machines etc. This thesis focuses on investigating the high temperature tribological behaviour of TiAlN, TiSiN and CrN coatings during sliding against high strength boron steel at low and elevated temperature (40ºC and 800ºC). The friction and wear studies were conducted by using a high temperature version of Optimol SRV test machine during sliding. 3D optical surface profiler (Wyko NT 1100) and SEM/EDS techniques were employed to analyse the test specimen surfaces in order to understand the nature of wear. In this work, the tribological properties of three different coatings of TiAlN, CrN, and TiSiN applied to tool steel substrates during sliding against high strength boron steel have been presented. At 40ºC, the friction characteristics of the three coatings are different. The friction coefficients of TiAlN, CrN, TiSiN coatings are µ=0.89 ±0.06, µ=1.05 ±0.17 and µ=1.1 ±0.09 respectively. The CrN coating has survived longer with steady coefficient of friction. TiAlN coatings shows the lowest friction, but its durability is shorter as compared to CrN coating and friction coefficient continued to increase as sliding progressed. TiSiN coating shows the highest coefficient of friction with short life time. Regarding the wear rate, CrN coating has very good wear resistance. Likewise, TiSiN coating has the poorest wear resistance, and was removed after a short time. At 800ºC, the coatings have been removed after short time, and the tribological properties were not influenced strongly by coatings. However, the coefficients of friction were higher, 1.28 ±0.2 of TiAlN, 1.29 ±0.1 of CrN, and 1.33 ±0.13 of TiSiN, compared to that at 40ºC. The wear modes observed are abrasive and adhesive wear at low and high temperatures.