**Yin-Yu Chang \* and Siddhant Amrutwar**

Department of Mechanical and Computer-Aided Engineering, National Formosa University, Yunlin 632, Taiwan; siddhant.amrutwar@gmail.com

**\*** Correspondence: yinyu@nfu.edu.tw; Tel.: +886-5-631-5332

Received: 19 February 2019; Accepted: 4 March 2019; Published: 7 March 2019

**Abstract:** Surface modification of steel has been reported to improve hardness and other mechanical properties, such as increase in resistance, for reducing plastic deformation, fatigue, and wear. Duplex surface treatment, such as a combination of plasma nitriding and physical vapor deposition, achieves superior mechanical properties and resistance to wear. In this study, the plasma nitriding process was conducted prior to the deposition of hard coatings on the SKH9 substrate. This process was done by a proper mixture of nitrogen/hydrogen gas at suitable duty cycle, pressure, and voltage with proper temperature. Later on, the deposition of gradient AlCrSiN coatings synthesized by a cathodic-arc deposition process was performed. During the deposition of AlCrSiN, CrN, AlCrN/CrN, and AlCrSiN/AlCrN were deposited as gradient interlayers to improve adhesion between the coatings and nitrided steels. A repetitive impact test (200k–400k times) was performed at room temperature and at high temperature (~500 ◦C) to assess impact resistance. The results showed that the tribological impact resistance for the synthesized AlCrSiN increased because of a progressive hardness support. The combination of plasma nitriding and AlCrSiN hard coatings is capable of increasing the life of molding dies and metal forging dies in mass production.

**Keywords:** duplex treatment; plasma nitriding; hard coating; tool steel
