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Investigation of the diamond machinability of newly developed hard coatings
Precision Engineering 24 (2000) 146 –152
Investigation of the diamond machinability of newly developed hard coatings R. Malza,*, E. Brinksmeiera, W. Preußa, J. Kohlscheenb, H.-R. Stockb, P. Mayrb a
Labor fu¨r Mikrozerspanung (LFM), University of Bremen, Badgasteiner Str. 2-3, D 28359 Bremen, Germany b Stiftung Institut fu¨r Werkstofftechnik (IWT), Bremen, Badgasteiner Str. 2-3, 28359 Bremen, Germany Received 28 June 1999; received in revised form 19 November 1999; accepted 19 November 1999
Abstract Presently, coatings of electroless nickel are used for diamond turning molds for injection molding of optical lenses. We have investigated the diamond machinability of substoichiometric hard nitride coatings (TiNx, TiAlNx, and CrNx). These coatings have a superior hardness compared to electroless nickel suggesting an improved wear resistance of molds with optical surface quality. In the case of CrNx and TiAlNx, high tool wear occurred, even after small cutting distances, and the surfaces showed a roughness larger than Ra ⫽ 0.5 m. A considerably higher surface quality was obtained on TiNx coatings. The best results (Ra ⫽ 15 nm) were achieved with a nitrogen content of x ⫽ 0.03. As a first application, a mold for a diffractive optical element was machined using this newly developed substoichiometric titanium nitride deposit. © 2000 Elsevier Science Inc. All rights reserved. Keywords: Hard coatings; Diamond machining; Injection molding
1. Introduction
2. Experimental procedure
Today, electroless nickel is used as a diamond machinable coating on steel parts for use in injection molding of Fresnel lenses. However, diamond machinable coatings with better abrasive properties are desirable. Although up to now nitride coatings (e.g., TiN) have been extensively studied for wear protection because of their high hardness, substoichiometric nitride coatings were investigated by comparably few researchers dealing mostly with fundamental properties [1– 4]. In this work, we investigate and propose a possible new application of substoichiometric nitride coatings not aiming at wear protection of cutting tools. On the contrary, the coatings are designed as machinable coatings for ultraprecision cutting with a monocrystalline diamond tool for use in injection molding of Fresnel lenses. Therefore, moderate hardness and coating smoothness are required. We analyzed composition, hardness, structure, adhesion, and adhesion improvement by a modified interlayer. Furthermore, the results of microcutting experiments are outlined.
Samples of 30-mm diameter and 8-mm thickness were made from HS 652 (M2) tool steel. Specimen surfaces were ground and preturned on a Hembrug Super-Mikroturn CNC precision lathe for turning tests. The coating deposition was performed with a DC magnetron sputter apparatus type Leybold Z 700 after a sample cleaning and bake-out. The deposition processes were performed with different nitrogen gas flows, while keeping the Argon flow constant. The average gas pressure was about 1 Pa. The target-to-substrate distance was 50 mm, and the deposition time was 100 min. Substrate temperature was found to be approximately 700 K as measured during a test coating with a thermocouple. Coating thickness was determined by the ball crater method. Chemical composition was measured by glow discharge optical emission spectroscopy (GDOS). Phase analysis was carried out by x-ray diffraction (XRD), and the analysis of the coating structure was performed by scanning electron microscopy (SEM) with a CamScan apparatus for TiNx and CrNx. Vickers hardness (HV) was determined by a Shimadzu HMV 2000 with a load of 0.5 N, thereby minimizing substrate effects. The coating adhesion to the substrate was measured by scratch tests. For investigation of the diamond machinability of the
* Corresponding author. Tel.: ⫹49 421 218 9449; fax: ⫹49 421 218 9441. E-mail address: [email protected] (R. Malz).
0141-6359/00/$ – see front matter © 2000 Elsevier Science Inc. All rights reserved. PII: S 0 1 4 1 - 6 3 5 9 ( 9 9 ) 0 0 0 3 8 - 0
R. Malz et al. / Precision Engineering 24 (2000) 146 –152
147
Fig. 1. Vickers hardness HV 0.05 vs. nitrogen content of coatings.
coatings, we performed face-turning experiments using single-crystal diamond tools with a nose radius of 3 mm. The experiments were carried out on a Hembrug Super-Mikroturn CNC lathe and on a Moore M18 Aspheric Generator. Spindle speed was varied between 100 and 500 rpm. The cutting and thrust forces were recorded with a Kistler 9257 B piezoelectric transducer. Tool wear was investigated with a SEM and by measuring the edge radius of the diamond with an atomic force microscope (AFM) [5]. Investigation of the surface topography and determination of roughness values were performed with a white light interferometer.
3. Results 3.1. Coating characterization The coating thickness was determined by the ball crater method and yielded about 20 m for all coatings. The chemical composition of the coatings was measured by GDOS. Fig. 1 contains these results together with hardness measurements. TiNx was investigated most intensively from x ⫽ 0 – 0.43, with an almost linear hardness increase from 550 to over 3000 HV 0.05. Fewer coating compositions were investigated for TiAlNx and CrNx. TiAlNx coatings show a lower hardness than TiNx in the range x ⫽ 0.05– 0.28. CrNx has a hardness of approximately 2000 HV 0.05 for x ⫽ 0.15– 0.36. The XRD analysis of TiNx coatings revealed the diffraction pattern of the hexagonal ␣-Ti phase with strong (002) orientation for low nitrogen contents (Fig. 2a). With increasing nitrogen content, the diffraction peaks are shifted toward lower diffraction angles (Fig. 2b,c). At a content of 15 at.% nitrogen, the diffraction pattern was characterized by the (111) oriented ⑀-Ti2N phase with broad peaks (Fig. 2d). Broad diffraction lines of the ␦-TiN phase become visible at 30 at.% (Fig. 2e). The TiAlNx coatings show a similar behavior as TiNx. CrNx coatings show the diffraction pattern of Cr2N. The
investigation of coating morphology of TiNx by SEM shows a columnar structure for nitrogen contents less than 2 at.% and above 30 at.%. Between these values, the coatings show no internal structure, as can be seen for 3 at.% nitrogen content in Fig. 3a. For TiAlNx coatings, SEM cross sections showed no internal structure for x ⬎ 0.07. CrNx coatings reveal a columnar structure in SEM cross sections (Fig. 3b). The critical loads as determined by scratch testing are shown in Fig. 4. Starting from a pure metallic coating with critical loads of 30 – 40 N, the adhesion falls off to approximately 10 –20 N, with a broad minimum around approximately 10 at.% nitrogen in the TiNx and TiAlNx case. Further increase of the nitrogen content in the coating leads to higher values for the critical load. Because of the inferior adhesion of the highly substoichiometric coatings, we tried an interlayer of either Ti or TiN for a TiNx coating with x ⫽ 0.03. This led to an enhanced adhesion, with critical loads of 30 N. 3.2. Diamond turning In case of TiAlNx and CrNx, diamond turning leads to a partial disruption of the hard coatings from the substrate exposing the feed marks of the preturning process on the steel substrate. SEM investigations of the used diamond tools revealed high abrasive tool wear. Considerably higher surface qualities were obtained on TiNx coatings. The influence of nitrogen content on the roughness Ra of the machined surface and the hardness of the coatings is shown in Fig. 5. After a cutting distance of 130 m, a roughness minimum was achieved for coatings with a nitrogen content of 3 at.%. In this case, the hardness of the coating is even superior to hardened steel. In some experiments, optical quality Ra ⬍ 10 nm was reached. No correlation between the Vickers-hardness of the coating and the achieved roughness value Ra was found. Investigations of the surface topography with a white light interferometer (WLI) revealed regions of well-defined feed marks alternating with regions where no feed marks
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Fig. 2. XRD diffraction patterns of TiNxcoatings with different nitrogen contents, a) 1 at.%; b) 5 at.% c) 10 at.%; d) 15 at.%; and e) 30 at.% nitrogen (default peaks of expected phases are also indicated.
could be recognized (cf. Fig. 6). This hints at a change of the cutting edge geometry during the turning process because of coating adherence at the cutting edge. SEM investigations of the cutting tool confirmed this. The cutting edge was completely covered by coating material. To characterize the diamond wear, the adhering coating material was removed by etching the tools in sulfuric acid. The wear mark on the clearance face was found to be of the same size as the extent of coating adherence. Experiments with reduced cutting speed in a range between 6 m/min and 30 m/min showed that the lowest cutting speed caused the smallest amount of adherence, and also resulted in the highest surface quality (cf. Fig. 7). Investigations of the cutting edge radius were performed with an AFM. For the machining of a TiNx coating (x ⫽ 0.03) the cutting edge radius increased from 50 to 720 nm for a cutting distance of 130 m. A cutting distance of 260 m caused an edge radius of more than 1 m (cf. Fig. 8).
3.3. Generation of microstructures The suitability of the new developed coating for generating microstructures was verified by turning Fresnel structures into a TiNx coating with a nitrogen content of x ⫽ 0.03. For these experiments, 5-m radius tools were used on a Moore M18 lathe. The Fresnel structures were investigated with a white light interferometer, and tool wear was characterized with a SEM. A medium cutting speed of 29 m/min led to a broken, fractured diamond tool, and a cutting speed of 6 m/min caused considerably lower wear. This corresponds with the results of the face-turning experiments (cf. Fig. 9). 4. Discussion From the Cr-N phase diagram, it is known that chromium cannot solve nearly any nitrogen at the process temperatures
Fig. 3. SEM cross sections of a) TiNx (x ⫽ 0.03); and b) CrNx (x ⫽ 0.15).
R. Malz et al. / Precision Engineering 24 (2000) 146 –152
Fig. 4. Critical loads of the investigated coatings vs. their nitrogen content.
dealt with here [6]. Therefore, as soon as some nitrogen is offered at deposition, the Cr2N phase forms, which is confirmed by XRD. This phase is known to be quite hard [4]. This property and the columnar structure prevent a good machining behavior, so that high abrasive tool wear is the consequence. In the TiAlNx lattice, nearly half of the titanium atoms
149
are replaced by the smaller aluminum atoms. This leads to reduced lattice distances, and for Al/Ti ⫽ 1 the phases found in the Ti-N system are maintained [7]. The presence of aluminum in the lattice may lead to a brittle behavior in the substoichiometric range. This could explain the lower adhesion values (Fig. 4) in comparison with TiNx, at least up to x ⫽ 0.05. Considering this, the inferior machining properties may be understood. According to the Ti-N phase diagram [8], the ␣-Ti lattice is able to solve a certain amount of nitrogen; for example, 3– 4 at.% at 800 K. Because PVD thermodynamically is a nonequilibrium process, the ␣-Ti lattice is forced to solve more nitrogen because of hindered mobility of the deposited particles, and an oversaturated metastable solution of nitrogen in titanium is formed [1]. The accommodation of the nitrogen atoms at interstitial ␣-Ti sites causes lattice widening, which explains the shift of the diffraction peaks in XRD to smaller angles (Fig. 2a– c) according to the Bragg equation. This was also observed by other groups [2]. At 15 and 30 at.%, the diffraction peaks become quite broad indicating—apart from high internal stress—small grain size. Grain-size decrease because of the oversaturated ␣-Ti
Fig. 5. Roughness Ra and hardness of the coatings for different nitrogen contents.
Fig. 6. Surface topography of a diamond turned TiNx coating (x ⫽ 0.03).
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Fig. 7. Coating adherence and tool wear for different cutting speeds.
lattice can well explain the vanishing columnar coating structure [2]. This is a well-known phenomenon for deposition of metastable phases, for example electroless nickel [9]. The hardness of the pure titanium coating is already relatively high because of stress and dislocations introduced by the bombarding particles during deposition. Increasing nitrogen content leads to further hardening by lattice dis-
tortion (Fig. 1). The ⑀-Ti2N and nitrogen-deficient ␦-TiN phase are known to show high hardness [1]. The minor adhesion of TiNx has also been observed by other authors [3]. Although the pure metallic titanium layer can absorb some shear stress during scratch test by plastic deformation, the stressed ␣-Ti(N) and ⑀-Ti2N phases cause lowered adhesion. The adhesion increases if the ␦-TiN phase at 30 at.% nitrogen is formed. Introducing a Ti or TiN
Fig. 8. AFM investigations of the cutting edge radius on a new and on used diamond tools.
R. Malz et al. / Precision Engineering 24 (2000) 146 –152
151
Fig. 9. Fresnel structure generated in a hard coating (left) and wear of diamond tools for different cutting speeds (center and right).
interlayer increases the adhesion, probably because of better lattice matching with the tool steel substrate [3]. A Ti interlayer was used by others to enhance adhesion of TiN coatings [10]. A TiN interlayer to improve adhesion of TiNx has not yet been reported. For the diamond turning of substoichiometric hard coatings, different wear mechanisms can be characterized. For TiNx coatings with a hardness between 1500 and 3000 HV 0.05 and for all investigated CrNx and TiAlNx coatings, high abrasive wear appeared after small cutting distances of less than 50 m. TiNx coatings with a lower hardness caused built-up cutting edges on the diamond tools. In this case, a continuous growing and detaching of coating adherence on the cutting edge leads to a variation of the tool geometry during the turning process, which results in an inhomogeneous surface topography. From the machining of pure metals, it is known that very low cutting speeds can minimize built-up cutting edges [11], which was also detected for the TiNx coatings. Cutting speeds of 6 m/min caused considerably lower coating adherence on the cutting edge than higher cutting speeds. Although wear of the diamond tool cannot be completely avoided, surfaces with optical quality were achieved, so that the new developed coating is suitable for highly wear-resistant die molds for optical elements. For the generation of microstructures, diamond tools with very small nose radii of e.g. 5 m must be used. These tools are susceptible to a very small amount of wear, so that Table 1 Results for the diamond turning of hard coatings
Coating morphology Major wear mechanism Surface condition
TiNx
CrNx
TiAlNx
Structureless
Columnar growth Abrasive wear Fractured
Structureless
Adhesive wear Smooth, optical quality
Abrasive wear Fractured
the machining of microstructures into a TiNx coating is limited to a few mm, until the diamond tool is worn.
5. Conclusion Substoichiometric coatings of TiNx, TiAlNx, and CrNx with 20-m thickness and varying nitrogen content were deposited by magnetron sputtering. The coatings were investigated concerning their chemical, structural, and mechanical properties. TiAlNx and CrNx led to poor surface quality and intolerable tool wear, which is explained by high hardness and columnar growth for CrNx and brittleness of TiAlNx. The best diamond-turning results were achieved with TiNx. Solving nitrogen into ␣-titanium leads to lattice widening and reduced grain size. This implies increasing hardness, structure smoothing, but also decreasing adhesion. The adhesion can be improved by an interlayer of either Ti or TiN. The best surface quality was obtained on TiNx coatings with a nitrogen content of x ⫽ 0.03 corresponding to a Vickers hardness of 1000 HV 0.05. Over a cutting distance of 130 m, a surface roughness Ra ⬍ 10 nm was reached. For this coating, adhesive wear of the diamond tools was observed, which could be reduced by decreasing cutting speed. Table 1 shows a summary of the results. For turning a Fresnel structure of 4-mm diameter into a TiNx coatings with a nitrogen content of x ⫽ 0.03, 5-m radius tools were used. In this case, we found a reduction of abrasive wear for the lower cutting speed. However, the remaining tool wear limits the diamond turnable diameter to a few millimeters. Therefore, future investigations focus on the optimization of the tool geometry and the machining parameters, for example, infeed and depth of cut.
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Acknowledgments This project was performed in cooperation with the Physikalisch Technische Bundesanstalt PTB Braunschweig and Berlin. The authors thank the Volkswagen Foundation for funding this research project.
References [1] Sundgren J–E, Johansson B-O, Karlsson S-E. Mechanisms of reactive sputtering of TiN and TiC II: Morphology and structure. Thin Sol Films 1983; 105:367– 84. [2] Elstner F, Ehrlich A, Giegengack H, Kupfer H, Richter F. Structure and properties of titanium nitride thin films deposited at low temperatures using direct current magnotron sputtering. J Vac Sci Technol A 1994;12:476 – 83. [3] Stanislav J, Sikac J, Cermak, M. Properties of magnetron-deposited polycrystalling TiN layers. Thin Sol Films 1990;191:255–73.
[4] Herr W, Matthes B, Broszeit E, Meyer M, Suchentrunk R. Influence of substrate material and deposition parameters on the structure, residual stree, hardness and adhesion of sputtered CrxNy hard coating. Surf Coat Technol 1993;60:428 –33. [5] Brinksmeier E, Malz R. Manufacturing and measurement of microstructures using ultraprecision machining and atomic force microscopy. Actuator 96, Proc 5th International Conference on New Actuators, Bremen, 26 –28 June 1996: pp. 92–5. ¨ ber[6] Holleck H. Bina¨re und Terna¨re Karbid— und Nitridsysteme der U gangsmetalle, Stuttgart/Berlin: Borntra¨ger, 1984. [7] Mu¨nz W. TiAlN films: a new alternative to TiN coatings. J Vac Sci Technol A 1986;4(6):2717–25. [8] Wriedt HA, Murray JL. Bull Alloy Phase Diags 1987;8(4):378. [9] Lu K, Sui ML, Lu¨ck R. Supersaturation of phosphorous in nanophase nickel crystallized from an amorphous alloy. Nanostruct Mat 1994; 4(4):465–73. [10] Schulz A, Stock H -R, Mayr P, Staeves J, Schmoeckel D. Deposition and investigation of TiN PVD coatings on cast steel forming tools. Surf Coat Technol 1997;94 –95:446 –50. [11] Vieregge G. Zerspanung der Eisenwerkstoffe. Du¨sseldorf: 2. Auflage, Verlag Stahleisen M.B.H., 1970.
Investigation of the diamond machinability of newly developed hard coatings R. Malza,*, E. Brinksmeiera, W. Preußa, J. Kohlscheenb, H.-R. Stockb, P. Mayrb a
Labor fu¨r Mikrozerspanung (LFM), University of Bremen, Badgasteiner Str. 2-3, D 28359 Bremen, Germany b Stiftung Institut fu¨r Werkstofftechnik (IWT), Bremen, Badgasteiner Str. 2-3, 28359 Bremen, Germany Received 28 June 1999; received in revised form 19 November 1999; accepted 19 November 1999
Abstract Presently, coatings of electroless nickel are used for diamond turning molds for injection molding of optical lenses. We have investigated the diamond machinability of substoichiometric hard nitride coatings (TiNx, TiAlNx, and CrNx). These coatings have a superior hardness compared to electroless nickel suggesting an improved wear resistance of molds with optical surface quality. In the case of CrNx and TiAlNx, high tool wear occurred, even after small cutting distances, and the surfaces showed a roughness larger than Ra ⫽ 0.5 m. A considerably higher surface quality was obtained on TiNx coatings. The best results (Ra ⫽ 15 nm) were achieved with a nitrogen content of x ⫽ 0.03. As a first application, a mold for a diffractive optical element was machined using this newly developed substoichiometric titanium nitride deposit. © 2000 Elsevier Science Inc. All rights reserved. Keywords: Hard coatings; Diamond machining; Injection molding
1. Introduction
2. Experimental procedure
Today, electroless nickel is used as a diamond machinable coating on steel parts for use in injection molding of Fresnel lenses. However, diamond machinable coatings with better abrasive properties are desirable. Although up to now nitride coatings (e.g., TiN) have been extensively studied for wear protection because of their high hardness, substoichiometric nitride coatings were investigated by comparably few researchers dealing mostly with fundamental properties [1– 4]. In this work, we investigate and propose a possible new application of substoichiometric nitride coatings not aiming at wear protection of cutting tools. On the contrary, the coatings are designed as machinable coatings for ultraprecision cutting with a monocrystalline diamond tool for use in injection molding of Fresnel lenses. Therefore, moderate hardness and coating smoothness are required. We analyzed composition, hardness, structure, adhesion, and adhesion improvement by a modified interlayer. Furthermore, the results of microcutting experiments are outlined.
Samples of 30-mm diameter and 8-mm thickness were made from HS 652 (M2) tool steel. Specimen surfaces were ground and preturned on a Hembrug Super-Mikroturn CNC precision lathe for turning tests. The coating deposition was performed with a DC magnetron sputter apparatus type Leybold Z 700 after a sample cleaning and bake-out. The deposition processes were performed with different nitrogen gas flows, while keeping the Argon flow constant. The average gas pressure was about 1 Pa. The target-to-substrate distance was 50 mm, and the deposition time was 100 min. Substrate temperature was found to be approximately 700 K as measured during a test coating with a thermocouple. Coating thickness was determined by the ball crater method. Chemical composition was measured by glow discharge optical emission spectroscopy (GDOS). Phase analysis was carried out by x-ray diffraction (XRD), and the analysis of the coating structure was performed by scanning electron microscopy (SEM) with a CamScan apparatus for TiNx and CrNx. Vickers hardness (HV) was determined by a Shimadzu HMV 2000 with a load of 0.5 N, thereby minimizing substrate effects. The coating adhesion to the substrate was measured by scratch tests. For investigation of the diamond machinability of the
* Corresponding author. Tel.: ⫹49 421 218 9449; fax: ⫹49 421 218 9441. E-mail address: [email protected] (R. Malz).
0141-6359/00/$ – see front matter © 2000 Elsevier Science Inc. All rights reserved. PII: S 0 1 4 1 - 6 3 5 9 ( 9 9 ) 0 0 0 3 8 - 0
R. Malz et al. / Precision Engineering 24 (2000) 146 –152
147
Fig. 1. Vickers hardness HV 0.05 vs. nitrogen content of coatings.
coatings, we performed face-turning experiments using single-crystal diamond tools with a nose radius of 3 mm. The experiments were carried out on a Hembrug Super-Mikroturn CNC lathe and on a Moore M18 Aspheric Generator. Spindle speed was varied between 100 and 500 rpm. The cutting and thrust forces were recorded with a Kistler 9257 B piezoelectric transducer. Tool wear was investigated with a SEM and by measuring the edge radius of the diamond with an atomic force microscope (AFM) [5]. Investigation of the surface topography and determination of roughness values were performed with a white light interferometer.
3. Results 3.1. Coating characterization The coating thickness was determined by the ball crater method and yielded about 20 m for all coatings. The chemical composition of the coatings was measured by GDOS. Fig. 1 contains these results together with hardness measurements. TiNx was investigated most intensively from x ⫽ 0 – 0.43, with an almost linear hardness increase from 550 to over 3000 HV 0.05. Fewer coating compositions were investigated for TiAlNx and CrNx. TiAlNx coatings show a lower hardness than TiNx in the range x ⫽ 0.05– 0.28. CrNx has a hardness of approximately 2000 HV 0.05 for x ⫽ 0.15– 0.36. The XRD analysis of TiNx coatings revealed the diffraction pattern of the hexagonal ␣-Ti phase with strong (002) orientation for low nitrogen contents (Fig. 2a). With increasing nitrogen content, the diffraction peaks are shifted toward lower diffraction angles (Fig. 2b,c). At a content of 15 at.% nitrogen, the diffraction pattern was characterized by the (111) oriented ⑀-Ti2N phase with broad peaks (Fig. 2d). Broad diffraction lines of the ␦-TiN phase become visible at 30 at.% (Fig. 2e). The TiAlNx coatings show a similar behavior as TiNx. CrNx coatings show the diffraction pattern of Cr2N. The
investigation of coating morphology of TiNx by SEM shows a columnar structure for nitrogen contents less than 2 at.% and above 30 at.%. Between these values, the coatings show no internal structure, as can be seen for 3 at.% nitrogen content in Fig. 3a. For TiAlNx coatings, SEM cross sections showed no internal structure for x ⬎ 0.07. CrNx coatings reveal a columnar structure in SEM cross sections (Fig. 3b). The critical loads as determined by scratch testing are shown in Fig. 4. Starting from a pure metallic coating with critical loads of 30 – 40 N, the adhesion falls off to approximately 10 –20 N, with a broad minimum around approximately 10 at.% nitrogen in the TiNx and TiAlNx case. Further increase of the nitrogen content in the coating leads to higher values for the critical load. Because of the inferior adhesion of the highly substoichiometric coatings, we tried an interlayer of either Ti or TiN for a TiNx coating with x ⫽ 0.03. This led to an enhanced adhesion, with critical loads of 30 N. 3.2. Diamond turning In case of TiAlNx and CrNx, diamond turning leads to a partial disruption of the hard coatings from the substrate exposing the feed marks of the preturning process on the steel substrate. SEM investigations of the used diamond tools revealed high abrasive tool wear. Considerably higher surface qualities were obtained on TiNx coatings. The influence of nitrogen content on the roughness Ra of the machined surface and the hardness of the coatings is shown in Fig. 5. After a cutting distance of 130 m, a roughness minimum was achieved for coatings with a nitrogen content of 3 at.%. In this case, the hardness of the coating is even superior to hardened steel. In some experiments, optical quality Ra ⬍ 10 nm was reached. No correlation between the Vickers-hardness of the coating and the achieved roughness value Ra was found. Investigations of the surface topography with a white light interferometer (WLI) revealed regions of well-defined feed marks alternating with regions where no feed marks
148
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Fig. 2. XRD diffraction patterns of TiNxcoatings with different nitrogen contents, a) 1 at.%; b) 5 at.% c) 10 at.%; d) 15 at.%; and e) 30 at.% nitrogen (default peaks of expected phases are also indicated.
could be recognized (cf. Fig. 6). This hints at a change of the cutting edge geometry during the turning process because of coating adherence at the cutting edge. SEM investigations of the cutting tool confirmed this. The cutting edge was completely covered by coating material. To characterize the diamond wear, the adhering coating material was removed by etching the tools in sulfuric acid. The wear mark on the clearance face was found to be of the same size as the extent of coating adherence. Experiments with reduced cutting speed in a range between 6 m/min and 30 m/min showed that the lowest cutting speed caused the smallest amount of adherence, and also resulted in the highest surface quality (cf. Fig. 7). Investigations of the cutting edge radius were performed with an AFM. For the machining of a TiNx coating (x ⫽ 0.03) the cutting edge radius increased from 50 to 720 nm for a cutting distance of 130 m. A cutting distance of 260 m caused an edge radius of more than 1 m (cf. Fig. 8).
3.3. Generation of microstructures The suitability of the new developed coating for generating microstructures was verified by turning Fresnel structures into a TiNx coating with a nitrogen content of x ⫽ 0.03. For these experiments, 5-m radius tools were used on a Moore M18 lathe. The Fresnel structures were investigated with a white light interferometer, and tool wear was characterized with a SEM. A medium cutting speed of 29 m/min led to a broken, fractured diamond tool, and a cutting speed of 6 m/min caused considerably lower wear. This corresponds with the results of the face-turning experiments (cf. Fig. 9). 4. Discussion From the Cr-N phase diagram, it is known that chromium cannot solve nearly any nitrogen at the process temperatures
Fig. 3. SEM cross sections of a) TiNx (x ⫽ 0.03); and b) CrNx (x ⫽ 0.15).
R. Malz et al. / Precision Engineering 24 (2000) 146 –152
Fig. 4. Critical loads of the investigated coatings vs. their nitrogen content.
dealt with here [6]. Therefore, as soon as some nitrogen is offered at deposition, the Cr2N phase forms, which is confirmed by XRD. This phase is known to be quite hard [4]. This property and the columnar structure prevent a good machining behavior, so that high abrasive tool wear is the consequence. In the TiAlNx lattice, nearly half of the titanium atoms
149
are replaced by the smaller aluminum atoms. This leads to reduced lattice distances, and for Al/Ti ⫽ 1 the phases found in the Ti-N system are maintained [7]. The presence of aluminum in the lattice may lead to a brittle behavior in the substoichiometric range. This could explain the lower adhesion values (Fig. 4) in comparison with TiNx, at least up to x ⫽ 0.05. Considering this, the inferior machining properties may be understood. According to the Ti-N phase diagram [8], the ␣-Ti lattice is able to solve a certain amount of nitrogen; for example, 3– 4 at.% at 800 K. Because PVD thermodynamically is a nonequilibrium process, the ␣-Ti lattice is forced to solve more nitrogen because of hindered mobility of the deposited particles, and an oversaturated metastable solution of nitrogen in titanium is formed [1]. The accommodation of the nitrogen atoms at interstitial ␣-Ti sites causes lattice widening, which explains the shift of the diffraction peaks in XRD to smaller angles (Fig. 2a– c) according to the Bragg equation. This was also observed by other groups [2]. At 15 and 30 at.%, the diffraction peaks become quite broad indicating—apart from high internal stress—small grain size. Grain-size decrease because of the oversaturated ␣-Ti
Fig. 5. Roughness Ra and hardness of the coatings for different nitrogen contents.
Fig. 6. Surface topography of a diamond turned TiNx coating (x ⫽ 0.03).
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Fig. 7. Coating adherence and tool wear for different cutting speeds.
lattice can well explain the vanishing columnar coating structure [2]. This is a well-known phenomenon for deposition of metastable phases, for example electroless nickel [9]. The hardness of the pure titanium coating is already relatively high because of stress and dislocations introduced by the bombarding particles during deposition. Increasing nitrogen content leads to further hardening by lattice dis-
tortion (Fig. 1). The ⑀-Ti2N and nitrogen-deficient ␦-TiN phase are known to show high hardness [1]. The minor adhesion of TiNx has also been observed by other authors [3]. Although the pure metallic titanium layer can absorb some shear stress during scratch test by plastic deformation, the stressed ␣-Ti(N) and ⑀-Ti2N phases cause lowered adhesion. The adhesion increases if the ␦-TiN phase at 30 at.% nitrogen is formed. Introducing a Ti or TiN
Fig. 8. AFM investigations of the cutting edge radius on a new and on used diamond tools.
R. Malz et al. / Precision Engineering 24 (2000) 146 –152
151
Fig. 9. Fresnel structure generated in a hard coating (left) and wear of diamond tools for different cutting speeds (center and right).
interlayer increases the adhesion, probably because of better lattice matching with the tool steel substrate [3]. A Ti interlayer was used by others to enhance adhesion of TiN coatings [10]. A TiN interlayer to improve adhesion of TiNx has not yet been reported. For the diamond turning of substoichiometric hard coatings, different wear mechanisms can be characterized. For TiNx coatings with a hardness between 1500 and 3000 HV 0.05 and for all investigated CrNx and TiAlNx coatings, high abrasive wear appeared after small cutting distances of less than 50 m. TiNx coatings with a lower hardness caused built-up cutting edges on the diamond tools. In this case, a continuous growing and detaching of coating adherence on the cutting edge leads to a variation of the tool geometry during the turning process, which results in an inhomogeneous surface topography. From the machining of pure metals, it is known that very low cutting speeds can minimize built-up cutting edges [11], which was also detected for the TiNx coatings. Cutting speeds of 6 m/min caused considerably lower coating adherence on the cutting edge than higher cutting speeds. Although wear of the diamond tool cannot be completely avoided, surfaces with optical quality were achieved, so that the new developed coating is suitable for highly wear-resistant die molds for optical elements. For the generation of microstructures, diamond tools with very small nose radii of e.g. 5 m must be used. These tools are susceptible to a very small amount of wear, so that Table 1 Results for the diamond turning of hard coatings
Coating morphology Major wear mechanism Surface condition
TiNx
CrNx
TiAlNx
Structureless
Columnar growth Abrasive wear Fractured
Structureless
Adhesive wear Smooth, optical quality
Abrasive wear Fractured
the machining of microstructures into a TiNx coating is limited to a few mm, until the diamond tool is worn.
5. Conclusion Substoichiometric coatings of TiNx, TiAlNx, and CrNx with 20-m thickness and varying nitrogen content were deposited by magnetron sputtering. The coatings were investigated concerning their chemical, structural, and mechanical properties. TiAlNx and CrNx led to poor surface quality and intolerable tool wear, which is explained by high hardness and columnar growth for CrNx and brittleness of TiAlNx. The best diamond-turning results were achieved with TiNx. Solving nitrogen into ␣-titanium leads to lattice widening and reduced grain size. This implies increasing hardness, structure smoothing, but also decreasing adhesion. The adhesion can be improved by an interlayer of either Ti or TiN. The best surface quality was obtained on TiNx coatings with a nitrogen content of x ⫽ 0.03 corresponding to a Vickers hardness of 1000 HV 0.05. Over a cutting distance of 130 m, a surface roughness Ra ⬍ 10 nm was reached. For this coating, adhesive wear of the diamond tools was observed, which could be reduced by decreasing cutting speed. Table 1 shows a summary of the results. For turning a Fresnel structure of 4-mm diameter into a TiNx coatings with a nitrogen content of x ⫽ 0.03, 5-m radius tools were used. In this case, we found a reduction of abrasive wear for the lower cutting speed. However, the remaining tool wear limits the diamond turnable diameter to a few millimeters. Therefore, future investigations focus on the optimization of the tool geometry and the machining parameters, for example, infeed and depth of cut.
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Acknowledgments This project was performed in cooperation with the Physikalisch Technische Bundesanstalt PTB Braunschweig and Berlin. The authors thank the Volkswagen Foundation for funding this research project.
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