## 1 Introduction  
Improving the wear resistance of tool steels is a pressing requirement. Better wear resistance means longer tool lifetime, which directly influences the production rate and ultimately the price of the manufactured products. Thus, high productivity requires tool steels with long lifetime and high performance. The required properties of tool steels are high strength at elevated temperatures, high impact strength, hot wear resistance, good toughness, and good hardenability. Despite high strength tool steels having good hot working capabilities, at elevated temperatures during the manufacturing process they suffer from surface oxidation, decarbonization, chemical interactions between oxides, biological contacts as well as thermal and mechanical loads [1, 2, 3]. High strength tool steels, such as 1.2344 (X40 CrMoV5-1), are widely used in hot-working processes (above 200 \({}^{\circ}\)C). Most commonly, this tool steel is used in the process of developing casting tools for light metal alloys for the automotive industry. To ensure consistent production quality at high volume productions, the surface roughness and wear properties of such tools need to be superior. Surface modification is commonly used to improve the wear resistance of tool steels. Tool steel 1.2344 was selected for this study because it fulfils the requirement of good hardenability at high temperatures before the surface modification. Common industrial processes to improve the wear resistance and the mechanical properties of the surface of tool steels are nitridation, surface hardening, laser surface treatment and surface coating [4]. Different research groups have shown the improvement possibilities of lifetime and performance of hot work tools by applying surface treatment processes, such as plasma nitriding, PVD coating or a combination of both treatments [5; 6; 7; 8]. During plasma nitridation, a nitride layer is formed on the surface of the used metal, which significantly improves the hardness of the surface [9]. PVD is used to develop layers of CrN, TiN, CrAlN or TiAlN depending on the chemical composition of the steel to improve surface hardness [10]. The improvement of PVD technology allows the development of multi-layer systems and the surface modification after nitridation [11]. Coated and heat-treated surface characterization was published by a different research group [12; 13; 14; 15]. The improvement of wear resistance can be attributed to the combined effects of surface roughness, surface hardness and stiffness. Based on fracture mechanics and crack propagation theories the damaging process usually starts with the failure of the surface. [16]. The authors found that with increasing surface roughness the fatigue limit of the surface layer declines. The literature discusses multiple methods to modify surface characteristic of different materials, however all the methods and characterization methodology are specially optimized to produce suitable results [17; 18; 19]. Further studies investigate the improvement possibilities of wear resistance of tool steel. These studies clearly indicate that surface toughness by itself does not provide a definite indication of wear resistance. Furthermore, to obtain the desired wear properties several layers are required with different stiffness and elasticity [20; 21; 22]. TiN monolayer deposited by PVD has inferior mechanical and wear properties compared to TiN/Ti or TiN/TiCN multilayers [23; 24]. The monolayers are more brittle than the multilayer systems and the damaging process starts sooner [24]. Thus, the development of a multilayer system by PVD to improve wear resistance is more advantageous. Tribological property investigation of TiAlN, CrAlN, Al\({}_{2}\)O\({}_{3}\), SiC and B\({}_{4}\)C PVD coatings have shown that the performance of the treated tools depends on several parameters such as substrate chemical composition and microstructure, substrate mechanical properties, deposition method and chemical composition of the deposited layer [25; 26; 27; 28; 29; 30; 31; 32; 33; 34].