**Abstract**

The chapter deals with the specific features concerning the application of wear-resistant coatings to improve the performance properties of ceramic cutting tools. The paper discusses the theoretical background associated with the specific operation conditions and wear of ceramic cutting tools and influencing the choice of the compositions and structures of wear-resistant coatings. The studies were focused on the application of the Ti-(Ti,Al)N-(Zr,Nb,Ti,Al)N multilayer composite coating with a nanostructured wear-resistant layer, as well as the (Cr,Al,Si)N– (DLC–Si)–DLC–(DLC–Si) and (Cr,Al,Si)N–DLC composite coatings in order to improve the cutting properties of ceramic tools. The chapter presents the results of the comparative cutting tests for the tools with the coatings under study, uncoated tools, and tools with the Ti-(Ti,Al)N commercial coating. The wear mechanisms typical for ceramic cutting tools with coatings of various compositions have been investigated.

**Keywords:** nanocomposite functional coating, diamond-like carbon (DLC), ceramic cutting tool, tool wear

### **1. Introduction**

Ceramic cutting tools are more and more widely used due to their high hardness, wear resistance, and relatively low cost [1–6]. The main specific feature of cutting ceramics is the absence of a binder phase, which significantly reduces the degree of softening in ceramic cutting tools during heating and increases their plastic strength. Due to the above, the cutting process can imply high cutting speeds, which significantly exceeds the cutting speeds typical for the machining with carbide cutting tools [1, 2, 5, 6]. While for a carbide cutting tool, the limiting level of cutting speeds is 500,600 m/min, then for a tool equipped with cutting ceramics, this level increases up to 9,001,000 m/min and higher [1]. However, the absence of the binder phase also has negative influence on the performance properties of ceramic cutting tools. In particular, their brittle strength, impact toughness, and resistance to crack formation decrease [1–4]. This fact significantly influences the wear patterns on ceramic cutting tools. For example, low crack resistance provokes the formation of a crack front, which, due to the absence of a plastic binder phase,

encounters no barriers to slow down or stop the crack development. The above is the main reason for micro- and macrochipping on contact pads of a ceramic cutting tool already at the stages of running-in or initial steady-state wear, causing failures because of brittle fracture. The noted wear mechanism prevails on ceramic cutting tools, and it actually does not depend on the cutting speed, because the temperature factor does not have a noticeable influence on the transformation of the wear mechanism. To a large extent, it is this mechanism of wear which determines the scope of application of the ceramic cutting tools [6–12].

At present, ceramic cutting tools are usually recommended for the finishing of gray, malleable, high-strength, and chilled cast irons, low- and high-alloy steels, including improved and heat-treated (up to HRC 55–60) non-ferrous alloys and structural polymer materials (K01 K05, P01 P05) [1–6]. Under the above conditions, tools equipped with ceramic cutting inserts are noticeably superior to carbide cutting tools in terms of working efficiency.

During the intermittent cutting, the use of ceramic tools in machining with advanced values of cross-section of cut (ap × f) sharply reduces their efficiency due to the high probability of sudden failure because of the brittle fracture of the cutting parts of the tools [1, 2]. This fact largely explains the relatively low volume of ceramic tools used in production sector [1, 2].

In this regard, the main direction of improving the performance activities of cutting ceramics is an enhancement of its strength characteristics to expand the area of the technological application in cutting. Recently, a new class of tool materials has appeared attributed to the group of cutting ceramics with increased strength, toughness, and crack resistance (silicon nitride, reinforced ceramics), and this fact indicates that the scope of application of ceramic cutting tools expands noticeably [1–6].
