coatings Article Development of DLC-Coated Solid SiAlON/TiN Ceramic End Mills for Nickel Alloy Machining: Problems and Prospects Sergey N. Grigoriev , Marina A. Volosova *, Sergey V. Fedorov, Anna A. Okunkova , Petr M. Pivkin , Pavel Y. Peretyagin and Artem Ershov Department of High-Efficiency Processing Technologies, Moscow State University of Technology “STANKIN”, Vadkovskiy per. 3A, 127055 Moscow, Russia; [email protected] (S.N.G.); [email protected] (S.V.F.); [email protected] (A.A.O.); [email protected] (P.M.P.); [email protected] (P.Y.P.); [email protected] (A.E.) * Correspondence: [email protected]; Tel.: +7-916-308-49-00 Abstract: The study is devoted to the development and testing of technological principles for the manufacture of solid end mills from ceramics based on a powder composition of α-SiAlON, β-SiAlON, and TiN additives, including spark plasma sintering powder composition, diamond sharpening of sintered ceramic blanks for shaping the cutting part of mills and deposition of anti- friction Si-containing diamond-like carbon (DLC) coatings in the final stage. A rational relationship between the components of the powder composition at spark plasma sintering was established. The influence of optimum temperature, which is the most critical sintering parameter, on ceramic samples’ basic physical and mechanical properties was investigated. DLC coatings’ role in changing the surface properties of ceramics based on SiAlON, such as microrelief, friction coefficient, et cetera, was studied. A comparative analysis of the efficiency of two tool options, such as developed samples of experimental mills made of SiAlON/TiN and commercial samples ceramic mills based on SiAlON, Citation: Grigoriev, S.N.; Volosova, doped with stabilizing additives containing Yb when processing nickel alloys (NiCr20TiAl alloy was M.A.; Fedorov, S.V.; Okunkova, A.A.; used as an example). DLC coatings’ contribution to the quantitative indicators of the durability of Pivkin, P.M.; Peretyagin, P.Y.; Ershov, ceramic mills and the surface quality of machined products made of nickel alloy is shown. A. Development of DLC-Coated Solid SiAlON/TiN Ceramic End Mills for Keywords: diamond-like carbon coating; high-speed milling; nickel alloy; SiAlON; spark plasma Nickel Alloy Machining: Problems sintering; roughness; wear resistance and Prospects. Coatings 2021, 11, 532. https://doi.org/10.3390/ coatings11050532 1. Introduction Received: 13 April 2021 At present, heat-resistant nickel alloys such as Inconel 718 type are widely used to Accepted: 28 April 2021 Published: 29 April 2021 manufacture critical parts operating under high thermal loads. For example, it can be noz- zles and working turbine blades, fairings, et cetera. The high-performance characteristics Publisher’s Note: MDPI stays neutral of parts made of nickel alloys determine the difficulties in their machining, accompanied with regard to jurisdictional claims in by increased heat and power loads on the cutting tool [1–3]. The machinability factor for published maps and institutional affil- high-temperature nickel alloys is in the order of 0.25 compared to the machining of C45 iations. steel (according to EN 10083-2: 2006). Simultaneously, solid carbide end mills are the most popular and versatile tool for machining aircraft parts. A modern approach to solving the problem of increasing machining nickel alloys’ productivity over the past years is the development and use for these purposes of end mills made of tool ceramics [4–6]. Today, large international companies, which are recognized leaders in cutting tools production Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. (Kennametal, Mitsubishi Materials, Iscar, and others), produce solid ceramic end mills on This article is an open access article an industrial scale [7–9]. Figure1a shows an example of industrial use of solid ceramic end distributed under the terms and mills made of SiAlON material for high-speed milling a turbine blade made of nickel alloy conditions of the Creative Commons on a multi-axis computer numerical control (CNC) machine. Attribution (CC BY) license (https:// The main advantage of using ceramic end mills when machining nickel alloys over creativecommons.org/licenses/by/ carbide tools is higher heat resistance. Nickel alloys of the Inconel type begin to soften ◦ 4.0/). at cutting temperatures of 800 C and above (Figure1b), after which significantly lower Coatings 2021, 11, 532. https://doi.org/10.3390/coatings11050532 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 532 2 of 27 power loads accompany their machining on the tool [1]. These temperatures correspond to cutting speeds above 350 m/min. Tool ceramics can be successfully operated due to their higher heat resistance in this high-speed milling mode. Sintered hard alloys at cutting temperatures over 800 ◦C lose their hardness rapidly (Figure1b), which excludes Coatings 2021, 11, 532 the possibility of their use in high-speed milling nickel alloys using the surface2 of layer28 plasticization effect [10,11]. (a) (b) FigureFigure 1. An 1. An example example of of industrial industrial use use of of SiAlON SiAlON solid solid ceramicceramic millsmills in high-speed milling milling of of a anickel nickel alloy alloy turbine turbine blade blade (a) and(a) and the the variation variation in in surface surface hardness hardness of of end end mills mills producedproduced from ceramics and and hard hard alloy alloy (b ()b )as as a afunction function of ofthe the cutting temperature. cutting temperature. PracticeThe main shows advantage that among of using a wide ceramic variety end mills of well-known when machining technical nickel ceramics alloys brandsover carbide tools is higher heat resistance. Nickel alloys of the Inconel type begin to soften at for tool purposes for the solid ceramic end mills manufacturing, the most suitable ceramic cutting temperatures of 800 °C and above (Figure 1b), after which significantly lower material is SiAlON (silicon-aluminum oxynitride), which belongs to the class of ceramics power loads accompany their machining on the tool [1]. These temperatures correspond based on silicon nitrides [12–15]. This ceramic consists of three or more phases: α-sialonic, to cutting speeds above 350 m/min. Tool ceramics can be successfully operated due to β-sialonic, and amorphous or partially crystallized grain-boundary phases. Sintered their higher heat resistance in this high-speed milling mode. Sintered hard alloys at cut- ceramics based on α/β-sialons are characterized by a unique combination of even higher ting temperatures over 800 °С lose their hardness rapidly (Figure 1b), which excludes the hardness than traditional silicon nitride while providing a high level of strength properties. possibility of their use in high-speed milling nickel alloys using the surface layer plastici- α β Thezation-sialon effect phase [10,11]. has a high hardness that is retained at elevated temperatures, and the - α β sialonPractice phase has shows a high that impact among strength a wide and variety fracture of well-known toughness. technical- and -sialonsceramics are brands ideally combined,for tool purposes and the for ratio the betweensolid ceramic these end phases mills canmanufacturing, be quite easily the variedmost suitable when preparingceramic powdermaterial compositions is SiAlON (silicon-aluminum (raw materials or oxynitride), precursors) which for subsequent belongs to sintering the class[ 4of– 6ceramics]. It makes itbased possible on silicon to obtain nitrides a different [12–15]. set This of ce physicalramic consists and mechanical of three or propertiesmore phases: of α the-sialonic, sintered ceramicβ-sialonic, necessary and amorphous for the cutting or partially tool’s specificcrystallized operating grain-boundary conditions phases. [16–19 Sintered]. ce- ramicsMoreover, based on any α/ ceramics,β-sialons are even characterized those obtained by a usingunique the combination most advanced of even technology, higher arehardness structurally than traditional inhomogeneous silicon nitride materials while that providing a priori a contain high level certain of strength defects proper- (such as micropores).ties. The α-sialon The sinteringphase hasprocess a high hardness of ceramic that powders is retained involves at elevated providing temperatures, an increased and temperaturethe β-sialon modephase has and a ishigh very impact technologically strength and complex. fracture toughness. In sintering, α- and grain β-sialons growth are can occur;ideally a significantcombined, and amount the ratio of residual between glassy these phasephases can can be be formed, quite easily which varied worsens when the hardnesspreparing and powder strength compositions of the material (raw during material high-temperatures or precursors) operation for subsequent [20,21 ].sintering Excessive porosity[4–6]. Itcan makes form it possible at a low to rate obtain of grain-boundary a different set of diffusion physical andand insufficientmechanical shrinkageproperties of theof powderthe sintered composition, ceramic necessary which sharplyfor the cutti worsensng tool’s the specific sintered operating ceramics’ conditions fracture [16– tough- ness19]. [22–24]. It should be borne in mind that a sintered ceramic workpiece, to obtain multi-edgeMoreover, tools, any must ceramics, be subjected even those to dimensional obtained using shaping the most processing advanced by technology, mechanical methods.are structurally One of inhomogeneous the