Surface and Coatings Technology 91-95 (1997) 597-602
coatings on carbide and cermet cutting tools
H.G. Prengel”,*, A.T. Santhanamb, R.M. Penichb, P.C. Jindalb, K.H. Wendt” “Kermmml Hertel AG, 90766 Fwth, Gewm> bKermarnetol Inc., L&robe, PA 15650, USA
Abstract Significant advances hav’e been made in the process design and development of PVD-TiAlN coatinefor carbide and cermet cutting tools. Higher plasma ionization in the TiAlN deposition process creates coatings with dense micfostructure and excellent adhesionchxacteristics. The resulting new generation of PVD-TiAlN coatings provide increased-productivity in a wide range of machining operations and workpiece materials. This paper discusses the characteristics of high-ionization TiAlN coatings and their performance in metal-cutting applications. 0 1997 Elsevier Science S.A. Keyn~ords:
coatings; Cermet; Carbide; Metal cutting
1. Introduction Continuously increasing demand for higher metalworking productivity is propelling the development of new manufacturing methods. Effective implementation of these new techniques, such as dry machining and hard machining, requires advanced carbide and cermet cutting tools [1,2]. The performance of advanced cutting tool materials can be enhanced by better and more wear resistant coatings; development of these new coatings shows a clear trend towards complex multi-component and multi-layer configurations. In CVD coatings, this trend is illustrated by the transition from CVD-TiC/TiCN/TiN coatings to complex and more wear-resistant CVD-A1203 coatings with controlled deposition of ~~-A1203or ~-A1203. Furthermore, CVD-A1202 can be combined with moderate-temperature-CVD TiCN coating to optimize the toughness of the CVD-coated products [3-51. PVD coatings are experiencing similar increases in complexity and wear resistance. Single layer PVD TiN coatings have been improved through partial or complete substitution of N with C (TiCN), B (TiBN, TiB2), or partial substitution of Ti with Al (TiAIN), Zr (TiZrN), or V (TiVN). These substitutions do not change the ordered face--centered cubic crystal structure, but provide a solid-solution strengthening effect that results in higher hardness and an associated + Corresponding author. 0257-X972/97/$17.00 PII
0 1997 Elsevier Science S.A. All rights reserved
increase in wear resistance. The substituting atoms can also impart higher chemical stability and improved oxidation resistance. These characteristics can in turn improve metal-cutting productivity by allowing the tool uber to increase machining speeds [6-91. TiAlN plays an increasingly important role in the design of advanced PVD coatings. While PVD-TiAlN coating5 have been known for more than 10 years, new high-ionization deposition processes permit coating properties to be controlled to produce optimum metal-cutting performance. This paper will describe the characteristics of new highionization PVD-TiAlN coatings and discuss their performance in metal-cutting.
2. TiAlN coating material TiAlN is an evolutionary development of the widely used hard coating material TiN. Although TiN coatings can be applied by thermal equilibrium chemical vapor deposition. TiAlN can only be produced using plasma processes. In plasma deposition, TiAlN is deposited as a metastable phase. Based on the composition of the target material, evaporation rate, and plasma ionization parameters, titanium can be substituted by aluminum to different levels [IO-U]. Fig. 1 shows an optical microstructure of a typical PVDTiAlN coating deposited on a cemented carbide substrate, aiong with an atom model for this coating material. X-Ray
H.G. Prengel et nl. /Sulfcire
and Coatings Technology 94-95
Table 1 Relative properties of PVD-TiAIN
Good chemical stability [5X9] High hot hardness Increased oxidation resistance [ 14, IS]
and their role in metal-cutting + + +
Provides better crater wear resistance Increases wear resistance Supports higher cutting temperatures/minimizes
diffraction analysis of this coating confirms an ordered facecentered cubic crystal structure with a lattice parameter smaller than that of PVD-TiN, implying that titanium is partially substituted by aluminum atoms (Al atoms are smaller than Ti atoms). The higher measured hardness of PVD-TiAlN (HVo,os of 2200-2500 kg/mm’) compared to TIN (HVo,os of 2000 kg/mm’) can be partially attributed to this solid solution strengthening effect of aluminum in the TiN crystal lattice. The microstructure of the coating also contributes to its hardness. Table 1 presents the relative properties of the PVDTiAlN and PVD-TiN coatings and their role in metal-cutting. When compared with TiN, the TiAlN coating material is endowed with a greater chemical stability [8,9], resistance to oxidation to higher temperatures [ 13,141, and higher hot hardness 1151. These characteristics provide advantages in metal-cutting, particularly at higher cutting speeds and when abrasive workpieces are machined.
3. Deposition coatings
of advanced PVD-TiAlN
In addition to coating composition, metal-cutting performance also depends on coating microstructure and adhesion to the substrate. For these key properties, type and control of
the coating process are of prime importance. This relationship is even stronger in the alloy PVD-TiAlN coatings described in this paper. PVD-TiAIN coatings are deposited mainly through reactive evaporation of Ti-Al target material using nitrogen. The basic evaporation methods for TiAlN deposition are sputter evaporation and arc evaporation. Due to the high power density of arc evaporation, the ionization level of an arc-Ti-Al plasma (ionization level approx. 50-90%) is higher than that of the Ti-Al plasma evaporated in a sputter process (ionization level approx. 5-10%) 116-181. Comparing these evaporation methods, the ion plating effectiveness of arc deposition is higher than that of conventional sputter techniques. Based on the increased effectiveness of arc processes, and the need for high quality ion-plating deposition, a new high-ionization sputter process for deposition of TiAlN coatings was developed. An enhanced plasma ionization method is an important step in designing the new high-ionization-TiAlN deposition process. Fig. 2 summarizes PVD-TiAIN deposition processes and shows that conventional sputter-TiAlN deposition is characterized by low ion density during deposition, resulting in a low-ion-plating effect. High ion density and the resulting high-ion-plating effect occur in the arc-TiAlN processes and the new high-ionization-TiAlN process. In addition to the ion-plating-deposition processes, optimized processes of
face centered cubic substitute mixed crystal ( solid solution )
Fig. 1. PVD-TiAIN
coating on cermet carbide (insert shows atom model).
et al. / S:ufim
md Coatings Techrzology 93-95
il ____------ ___-_-_ I t II Enhanced
Fig. 2. PVD deposition processes for TiAlN
heating and plasma etching are necessary befor,e starting TiAlN deposition. These pre-coating process steps mainly improve the adhesion characteristics and help errsure highquality deposition of advanced TiAlN coatings (Fig. 3). Scanning electron micrographs (SEM) of TiAlPd coatings deposited by the conventional sputter process, high-ionization process, and arc process reveal different microstructural characteristics (Fig. 4). Conventional sputter-TiAlN coatings (Fig. 4a) exhibit an open and columnar structure. PVD-TiAIN coatings produced with high ion-plating effect possess a denser microstructure. The morphology of highionization-TiAlN-coatings (Fig. 4b) shows a homogeneous, pore-free and compact microstructure with smooth coating surface. Investigations of single layer arc-TiAlN coatings (Fig. 4c) also show the dense ion plating structure of the coating. Compared with the high-ionization-TihlN coatings, arc-TiAlN coatings also have a uniform and porefree interface. A typical feature of arc evaporated TiAlN coatings is the presence, in varying amounts, of macropartitles (droplets). Based on their characteristic ion-plating appearance and uniform coating thickness, in combination with excellent adhesion (critical load Lc > 60 N), both advanced PVDTiAlN coatings (arc-TiAlN and high-ionization-TiAlN) offer high performance potential in metal-cutting applications.
4. Metal-cutting coatings
Carbide and cermet tool inserts in turning and milling style geometries were coated with conventional sputtered or high-ionization sputtered TiAlN. Some tools were also coated with single layer arc-TiAlN and ion-plated TIN coatings. The coated tools were tested in milling, turning, and drilling of various workpiece materials and their relative performances were evaluated.
4.1.1. Alloy steel Cmbide tools. SEHW 43A6T style inserts were produced from a WC-2% TaC-11.5% Co alloy and were coated with either TiAlN (conventional sputtering and high ionization sputtering) or TIN (ion plating process). The coated inserts were evaluated in a wet milling application on 4140 steel (hardness 190-220 BHN) at 152 m/min (500 sfm), 0.25 mm/rev/tooth (0.010 ipt), and 2.5 mm dot (0.100 inch). The milling cutter produced a 76.2-mm (3-inch) wide cut and traveled 609.6 mm (24 inches) per pass. Fig. 5 shows the progression of maximum wear as a function of length of material milled (number of passes) for the three tool materials test-ed. Themaximum wear comprises the abrasive flank wear component as well as microchipping commonly observed on tool edges during milling operations. Conventional sputtered TiAlN coating material exhibits the highest wear rate. The lowest wear behavior and the longest tool life are exhibited by-the high-ionization process coating. The ion-plated TIN shows intermediate wear behavior. Cemet tools. SEAW 1204AFN style inserts were manufactured from a titanium carbonjtride cermet grade (18% binder (nickel + cobalt)) and were coated with either single layer arc-TiAlN or high-ionization sputtered TiAIN. Both uncoated and coated ims were evaluattidry milling of AISI 6150 steel at 302 m/mm (990 sfm), 0.10 mm/rev/tooth (0.004 ipt), and 2.0 mm dot (0.080 inch). Fig. 6 shows the wear curves observed. The pronounced increase in the maximum flank wear resistwc&ols relative to the uncoated cermet insert is readily apparent. The wear curves of the arc-TiAlN and the high-ionization sputtered TiAlN are nearly identical. 4.1.2. Ductile cast iron SPG432 style inserts were manufactured from a WC3.5% TaC-6% Co alloy and~w!ele_~coatehwith~ PVDTiAlN through a conventional sputtering process or the high ionization process. The inserts-were- evaluated indry milling of ductile cast iron block (80-55-06 material) in which a number of holes had been drilled to provide interruptions during machining. The machining conditions were as follows: 305 m/min (1000 sfm), 0.13 mm/rev/tooth
4 a Optimized
. High ion-plating
adhesion ion-plating-structure 4
PVD TiAlN coatings
Fig. 3. Process design of advanced PVD-TiAIN
H.G. Prengei et al. /Surfme
Conventional ( a ) Stwtter-TiAlN
Fig. 4. Typical microsiructure
and Coatings Technology 94-95
( c ) Arc-TiAlN
( b ) Hinh-Ionization-TiAIN
coatings deposited by: (a) Conventional
(0.005 ipt), and 2.0 mm dot (0.080 inch). On this workpiece material, flank wear was the dominant tool failure mechanism. Fig. 7 shows flank wear as a function of the length of workpiece material milled (passes) for the two tool materials evaluated. The wear rate was similar for both inserts until 20 passes, at which time the tool with conventionally sputtered TiAlN coating began to show premature chipping and accelerated wear. 4.2. Twni,zg application CNGP 432 style inserts with sharp edges were produced from a fine grained WC-6% Co alloy and were coated with either ion-plated PVD TiN, conventional sputtered TiAlN, or high-ionization sputtered TiAlN. The coated inserts were evaluated in turning (with coolant) of 304 stainless-steel bar at 244 mfmin (800 sfm), 0.2 mm/rev (0.008 ipr), and 1.5 mm dot (0.060 inch). The dominant tool failure mode was flank wear. Fig. 8 shows the wear curves. The superior performance of the high ionization sputtered TiAlN compared to other tool inserts may be noted.
sputter process; ib) high-Ionization
PVD-TiN-coated solid carbide drills have shown considerabIe tooi life advantage over uncoated driils due to the high wear resistance of the TiN coating. PVD coatings are particularly suitable for solid drills with complex cutting geometry designs because the PVD process maintains the stability of the cutting edge while retaining the strength of the carbide substrate, Additional tool life advantage can be expected from TiAlN coating because of its higher hardness and wear resistance. Drilling tests were performed on GG30 (Class 40B) gray cast iron at 120 m/min (396 sfm) and 160 m/min (528 sfm), a feed rate of 0.25 mm/rev (0.010 ipr), and a depth of 40 mm (1.67 inch) with a WC-6% Co solid carbide tool with and without high-ionization sputtered TiAlN coating. Fig. 9 shows the tool life data in terms of the number of holes drilled. The remarkable consistency of the TiAlN-coated drills compared to the uncoated drills can be attributed to the excellent adhesion and the fine-grained microstructure of the high-ionization sputtered TiAlN coating.
Fig. 5. Performance of conventional sputter-TiAlN and high-ionizalionsputter-TiAlN vs. ion-plating-TiN in milling 3130 steel.
process; (c) arc evaporation.
Fig. 6. Performance of arc-RAIN and high-ionization-sputter-TiAlN cermet in milling AISI 6150 steel.
( Number of passes, 24 inch per pass )
Fig. 7. Performance of conventional sputter-TiAIN and high-Ionizationsputter-TiAlN vs. ion-plating-TiN in milling SO-5506 ductile-iron.
Fig. 9. Performance gray cast iron.
5. Discussion High-ionization sputtered TiAlN coated tools offer improved metal-cutting performance over conventionally sputtered material across a broad range of workpiece materials, from long-chipping alloy steel, long-chipping and high work-hardening 304 stainless steel, to the relatively shortchipping and abrasive gray and ductile cast iron. This improvement can be ascribed to several factors. Foremost among them is the consistently high adhesion strength of the TiAlN coating produced by the high-ionization sputtering process. Another salient feature of the coating is its dense microstructure, free from interfacial pores or columnar features that are frequently observed with the conventional sputtered processes. These characteristics of the high-ionization sputtered material confer resistance to coating flaking and chipping and provide improved abrasive wear resistance, leading to longer tool lives. As expected, the TiAlN coating produced by the arc process also performed well due to its dense microstructure. The significantly better performance of the high-ionization sputtered TiAlN compared to the ion-plated TiN coating does not arise from any microstructural difference (both have dense microstructures), but is associated with the solid solution strengthening (greater flank wear resistance),
greater chemical stability and oxidation resistance (less tendency for crater wear), and higher hot hardness (higher resistance to abrasive wear) of the TiAlN coating material.
6. Conclusions It has been shown that a microstructurally dense, highly adherent PVD-TiAlN coating can be deposited by a highionization sputtering process. Metal-cutting tests performed in turning, milling, and drilling of several workpiece materials demonstrate the superiority of high-ionization sputtered TiAlN coating over the conventional sputtered TiAlN or ion-plated TIN coating on carbide and cermet cutting tools.
Acknowledgements The authors gratefully acknowledge the assistance of Gtinter Roder, Dirk Kammermeier, and Al Shuster with the metal-cutting tests conducted in this study. Some PVD-TiAlI\Tcoatings were provided by CemeCon GmbH.
Cutting Time ( minutes )
Fig. 8. Performance of conventional sputter-TiAlN and high-ionizationsputter-TiAlN vs. ion-plating-TiN in turning 304 stainless steel.
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