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A novel cBN composite coating design and machine testing: A case study in turning
Surface & Coatings Technology 206 (2011) 273–279
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
A novel cBN composite coating design and machine testing: A case study in turning Wenping Jiang a, Ajay P. Malshe a, b,⁎ a b
NanoMech Inc., Springdale, AR 72764, United States Materials Research and Manufacturing Laboratories, Department of Mechanical Engineering, University of Arkansas, Fayetteville, AR 72701, United States
a r t i c l e
i n f o
Article history: Received 15 February 2011 Accepted in revised form 4 July 2011 Available online 13 July 2011 Keywords: Cubic boron nitride Composite coating Deposition Characterization Machining performance Tool wear
a b s t r a c t A composite coating series based on nano- and micro-sized cubic born nitride particulates and choice of application-specific binders were developed for turning engineering materials. The coating series were produced via two sequential processes: electrostatic spray coating of cubic boron nitride particles with size less than 2 μm for a conformal porous coating preform of designed thickness; and chemical vapor infiltration of ceramic binder phase(s) at a temperature of around 1000 °C for a dense and well adherent composite coating. In this paper, the coating design for different applications is discussed. As a study case, cubic boron nitride–titanium nitride composite coating was characterized by use of different techniques for coating cross-section, elemental composition, crystal phases, and adhesion strength. Characterization results indicated a composite coating with uniform coating thickness and evenly distributed cubic boron nitride particles in a titanium nitride matrix. Additionally, the coating was tested for its machining performance in continuous turning of AISI 4340 hardened steels and AISI 4140 pre-hardened steels at representative application conditions, and compared to corresponding industrial benchmarks. Testing results showed that the composite coating outperforms its industrial counterparts, polycrystalline cubic boron nitride compacts, titanium aluminum nitride coating by physical vapor deposition, multi-layer coating by chemical vapor deposition, and aluminum oxide bulk tools, in their respective applications. © 2011 Published by Elsevier B.V.
1. Introduction Cubic boron nitride (cBN) in bulk form and as a coating material is of significant interest especially for cutting tools due to its high hardness, thermal stability and chemical inertness up to 1200 °C in machining ferrous materials [1]. In cutting tools, cBN is mainly used as bulk or tipped form, and possesses many of the desirable properties for cutting tools. Thus, cBN is one of the favorable tool material choices for machining engineering materials such as hardened steel, pre-hardened steel, and cast irons, in spite of its high cost and limited geometry options resulting from its fabrication, which uses a high-pressure and high-temperature (HP–HT) process. Driven by the need for more options for application-specific tools such as inserts with chip breakers and round tools, and the need for higher productivity, development effort has been reported in exploring cBN coatings or thin films for cutting tool applications [2,3]. Various physical and chemical vapor deposition (PVD and CVD, respectively) techniques have been utilized for the deposition of cBN films [4–13], and been able to deposit nearly phase-pure adherent cBN films up to 2 μm for cutting tools. Keunecke et al. [2] reported a 1.2 μm
⁎ Corresponding author at: NanoMech Inc., Springdale, AR 72764, United States. E-mail address: [email protected] (A.P. Malshe). 0257-8972/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2011.07.008
thick cBN film on TiN and TiAlN pre-coated cemented carbides with a thin B-C-N gradient layer (in the range of 100–200 nm) deposited using a radio-frequency diode sputtering setup. The cBN deposited inserts demonstrated significantly extended tool life in turning H13 steel (~52 HRC) and milling alloyed tool steel. Chong et al. [14] synthesized a 2.0 μm thick cBN film on diamond pre-coated cemented carbide by using an electron cyclotron resonance microwave plasma-chemical vapor deposition process. The superior chemical resistance and extreme hardness from the combination of diamond transitional layer and cBN film may have potential for machining steels and ferrous materials. Clearly, these techniques have demonstrated the potential to provide economic alternatives to the high temperature/high pressure synthesis techniques. However, issues including (1) high compressive stress (in the order of a few GPa up to 20 GPa) existing unfavorably at the substrate–coating interface and leading to the delamination of the coating from the substrate [15], (2) low deposition rate [11], and (3) existence of hexagonal boron nitride (h-BN) and/or turbostratic boron nitride (t-BN) in the synthesized film [15], where the non-cubic phases are soft and contain defects such as boron dangling bonds, leading to mechanical destabilization of the coating or film, are still the major hindrances associated with the growth of cBN films in desirable thicknesses for machining applications. An advancement in synthesizing thick (5–20 μm) cBN based composite coatings has been made for machining applications
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[16–18]. It starts with electrostatic spray coating (ESC) of presynthesized cBN crystals as a porous powder coating preform, followed by chemical vapor infiltration (CVI) of ceramic phases such as TiN, TiCN, etc. as a binder to form an adherent composite coating. In the work reported here, the coating design needed for different applications was discussed. As a case study, a cubic boron nitride– titanium nitride composite coating was characterized by use of different techniques for examining the coating cross-section, elemental composition, crystal phases, and adhesion strength. Further, machining test results in continuous turning of AISI 4340 hardened steel and AISI 4140 pre-hardened steel were presented and compared to performance data from industrial benchmarks.
2. Experimental 2.1. Coating deposition process The coating was synthesized by a two-step sequential process starting with the deposition of pre-synthesized cBN powder by using ESC to form a porous coating preform of desired thickness and pore size, followed by CVI of ceramic phases as a binder for achieving an adherent coating to the substrate. The ESC deposited cBN powder coating preform loosely adheres to the substrate by electrostatic forces, and largely controls the coating surface smoothness or morphology [19], while CVI is critical to ensure fully dense coating. For a tool surface with a porous cBN particulate layer, infiltration should begin at the substrate surface, and proceed toward the top of the coating preform, instead of the other directions because, in the infiltration process, formation of dense film may lead to incomplete diffusion of the ceramic binder, leaving behind unfilled pores that are detrimental to coating performance. Thus, it is necessary to control the infiltration process in such a way that the diffusion rate of gaseous phases is higher than the reaction rate. The details of the coating process can be found elsewhere [16–18].
2.2. Characterization of the coating The coating thickness was measured by using a standard ball crater tester, which rotates a carbide ball (diameter of 20 mm) with a paste of 0.1 μm nanocrystalline diamond particles, and was confirmed by optical characterization of the cross-section of the coating. In ball crater testing, the ball rotating against the specimen surface creates a circular wear track as deep as the coating thickness. The radius of the track is then measured by using optical microscopy. Thus, the thickness of the coating can be derived by following the related geometrical equation. cBN particle distribution in the composite coating was characterized using scanning electron microscopy (SEM, XL30, Phillips). The surface roughness (Ra) of the coating was measured with a surface profilometer (Veeco Dektak 3030). X-ray diffraction equipment (XRD, Philips X'Pert dual goniometer diffractometer) was used to analyze the material phases in the coating. For comparison, XRD analysis was also applied to the aspurchased cBN powder at the same XRD settings. The adhesion of the coating to the substrate was evaluated by the use of the Rockwell C indentation method on a conventional hardness tester (ADR-12). The load applied on the indenter was 100 kgf. The adhesion between the coating and the substrate was assessed based on observation of the radial and peripheral cracks. Additionally, a scratch test was performed by running a Rockwell C diamond indenter (120° cone with 200 μm radius hemispherical tip) across the crater created by the ball crater tester and followed by optical observation of the scratch to quantify critical load. These analyses provide useful insight into the behavior of the coating for turning AISI 4340 hardened steel and AISI 4140 pre-hardened steel.
2.3. Machining tests Wear resistance of the composite coating was evaluated by conducting continuous turning tests on AISI 4340 hardened steel and AISI 4140 pre-hardened steel at typical machining conditions recommended by tool manufacturers for each specific material. In turning AISI 4340 hardened steel, CNMA432 inserts were used, while in turning AISI 4140 pre-hardened steel, CNMG432 inserts were used. Table 1 lists the details of workpiece materials, machining conditions, benchmark inserts, and tool failure criteria used in the machining tests. In the testing process, tools were inspected under a toolmakers microscope at regular intervals for their wear developed at different locations, which include flank surface, nose, and rake surface. The tool life is defined as the time corresponding to the given criterion (Table 1). The tool performance of the composite coating coated inserts was compared to that of the industrial benchmark tools at identical conditions. The tested inserts were further analyzed by using optical microscope and SEM for their failure modes. 3. Results and discussion 3.1. Coating design for machining of different materials A general requirement for coatings on cutting tools is to provide the desirable properties, which include (A) strong adhesion to tool substrate, (B) high hardness and hot hardness, (C) strong abrasive wear resistance, (D) high facture toughness; (E) high chemical stability in contact with workpiece material at high contact pressure and elevated temperature; and (F) low thermal conductivity and friction coefficient [20]. Depending on specific workpiece material and machining conditions, the above desirable properties can be achieved by tailoring coating thickness, different binders, and adjusting volumetric ratio of cBN in the ceramic binder matrix. In reference to the typical applications of PCBN tools, and the recommended applications for PVD and CVD mono-layer and multiple-layer coatings, a snapshot of the composite coating design is listed in Table 2. For a specified application, an effective solution is the combination of substrate, edge preparation, and suitable coating. Especially, good adhesion between the coating and substrate is critical. Thus, matching of thermal expansion coefficient (TEC) between the coating material and substrate, in addition to chemical compatibility, is preferable especially for CVI at a temperature approaching 1000 °C. Otherwise, an interfacial layer or compositionally graded layer is needed to minimize possible stress build-up, which is considered one of the major causes for coating delamination. In this case, cBN has a TEC in the vicinity of 5–6 × 10− 6/K; TiN has a TEC of 9.4× 10− 6/K. If 45% (volumetric) of cBN particles is in the TiN matrix, then the TEC of the composite is estimated as 7.5 × 10− 6/K by taking the respective volumetric ratio as a weighing factor. Therefore, a generic C2 grade is preferred because it has a TEC of 5–7 × 10 − 6/K, close to that of cBN–TiN. If TiCN (TEC = 8.4 × 10− 6/K) is used as an infiltrant with the same cBN ratio as cBN in TiN, then the TEC of the composite (cBN–TiCN) will be around 6.8 × 10 − 6/K, which is even closer to the TEC of WC–Co. Based on the above consideration, for general continuous turning applications, a coating thickness of 10–15 μm was selected to meet adhesion and other general application requirements. The selected generic C2 grade (~6.0%) has micron size WC grains for balanced hardness and toughness. The edge of the insert was just de-burred without honing. While for machining with interruptions such as oil holes or key ways present on workpieces, the selection of substrate, edge preparation, and infiltrants will be different from those recommended for continuous machining. In this case, toughness of the combination of coating and substrate is critical, in addition to hardness for wear resistance. Therefore, substrates with high Co percentage (typical 7.5% or above is preferred) and edge preparation is needed. The edge hone varies
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Table 1 Workpiece materials, machining conditions, benchmark inserts, and tool failure criterion used in machining tests. Workpiece materials
Machining conditions
Benchmark inserts
AISI 4340 hardened steel; hardness: 50–52 HRC; D = 62.5 mm (2.5"), L = 254 mm (10.0")
Vc = 150 mm/min f = 0.15 mm/rev αp = 0.25 mm with cutting fluid
ISO 3685, flank wear of 0.2 mm (0.008") A) Polycrystalline cubic boron nitride or surface finish significant degradation (PCBN with CBN of 55% in volume, CNMA432) B) PVD TiAlN coated CNMA432 (~4 μm) C) CVD TiCN–Al2O3–TiN coated CNMA432 (~15 μm); D) Bulk Al2O3 CNMA432 (with 30% of TiC) CVD TiCN–Al2O3–TiN coated CNMG432 (~15 μm) ISO 3685, flank wear of 0.2 mm (0.008")
AISI 4140 pre-hardened steel; hardness: 25–32 HRC; Vc = 183 m/min; D = 62.5 mm (2.5"), L = 254 mm (10.0") f = 0.41 mm/rev αp = 1.78 mm with cutting fluid
according to degree of interruptions. For severe interruptions, slightly large hone (N0.025 mm) is chosen to avoid edge chip-off. In terms of coating, combination of a specific density of cBN with different infiltrants such as TiCN and Al2O3 is suggested. Overall coating thickness is typically limited to 10 μm. Machining tests on 4340 hardened steel bars with a key way of 9.5 mm (3/8") in longitudinal direction using the composite coating coated CNMA432 inserts of high Co percentage have demonstrated tool life being superior to that of similar inserts with CVD multiple-layer coating (with Al2O3) on Co enriched carbide from major tool manufacturers. Details of the coating and machining tests with interruptions will be presented in the future.
3.2. Porous coating formation by electrostatic spray coating With a given average particle size of cBN powder (b2.0 μm), the deposition of cBN particles using ESC is affected by the physical properties (particle density, particle shape, particle size distribution, and electrical resistivity) of cBN powder, in addition to process parameters including electrode–substrate distance, electrical voltage applied at the electrode, and main air pressure. In the ESC process, aerodynamic forces and electrostatic forces (both attractive and repulsive forces) largely govern the deposition of the charged particles. In regions relatively far away from the grounded substrate, aerodynamic force dominates; electrostatic forces play an influential role in regions close to the substrate. As the cBN powder layer is built up, it starts to form a mirror electrostatic field to deter the coming of more charged particles. Thus, the formation of a porous coating preform is the result of attractive force and repulsive force. The balance of these interacting forces determines the maximum possible thickness of a coating preform for a given powder material at a specified deposition condition. Based on the given cBN particles with average particle size of 0.6 μm and typical cluster size of 5 μm, and an optimized condition (voltage: −60 kV; substrate–electrode distance: 150 mm) [21], the coating preform thickness could be as thick as 14 μm, as depicted in Fig. 1. The details of the modeling process for the attractive force and repulsive force for charged cBN particles can be found in [19]. A typical cross-section thickness profile and the surface morphology of the coating preform are shown in Fig. 2(A) and (B), respectively.
Table 2 A snapshot of cBN based composite coating design for different engineering materials. Infiltrants Coating form
Potential applications
TiN TiCN TiCN– Al2O3 HfN
Low carbon steel, alloy steel (partially), and nodular cast iron Alloy steel, ductile iron, cast iron, and stainless steel Hardened steel, tool steels, stainless steel, titanium alloy (partially), and Ni-based super alloy Hardened steel, and all other above materials
cBN–TiN cBN–TiCN cBN– TiCN/Al2O3 cBN–HfN
Tool failure criteria
3.3. Coating structure Fig. 3 illustrates the surface morphology and cross-section of the cBN based composite coating with TiN, TiCN, and ZrCN infiltrants, respectively. For the TiN infiltration, the surface has relatively fine grain size with features showing facets of cubic structures [Fig. 3(A)]; the cross-section clearly indicates two distinct layers, a cBN particulate based composite layer (~13 μm) with particles in a reasonably uniform distribution across the TiN matrix and a pure TiN layer (about 3 μm) on top of the composite layer [Fig. 3(B)]. The thickness is consistent with the average thickness, 15± 1.0 μm, obtained from ball crater testing. The cBN particle density in this case is about 40–45%, but can be tailored by slightly changing the particle deposition process. The thickness of the pure TiN layer can be adjusted by modifying the process time in the infiltration process. Elemental composition analysis by using EDS on the coating surface indicated a Ti/N ratio of 0.89. This ratio was reduced to 0.52 in the composite coating due to the contribution of N from cBN. For TiCN infiltration, the surface shows similar morphology as that of TiN infiltration, but with relatively coarse grain [Fig. 3(C)] due to the fairly high temperature (N1030 °C) associated with the infiltration process. The cBN particles are well “locked” in the TiCN matrix, as shown in Fig. 3(D). TiCN has a thermal expansion coefficient closer to cBN than TiN, thus, the tendency for crack initiation is less than with TiN. Fig. 3(E) illustrates the surface morphology of a ZrCN infiltrated composite coating, which demonstrates nodules consisting of many fine needle-shaped crystals. The nodules are more or less related to the cBN particle clusters or agglomerates, while lenticules are from the ZrCN. A high magnification SEM image shows “mushy” zones around the cBN particles, which are integrated to the ZrCN matrix for adhesion [Fig. 3(F)]. The surface roughness (Ra) of the coating with different infiltrants is in the range of 3–5 μm. Depending on the requirements for different applications, the composite coating can be tailored to as thin as a few microns with relatively high density of cBN particles. Additionally, the density of cBN particles can be adjusted with different particle volumetric ratio. Fig. 4 illustrates the sample cross-sections of thin and high cBN density composite coatings, respectively. As shown in Fig. 4(A), the overall coating thickness (including capping layer) is about 4 μm with cBN–TiN composite layer of about 1.0 μm. Capping layer, TiN, stacked on the top of cBN–TiN forms a unique layered structure, which is important for machining specifically with interruptions. Fig. 4(B) is a cross section of the composite coating with up to 70% (volumetric) cBN particles. The high cBN density significantly enhances the wear resistance of the coating and modifies the thermal conductivity of the composite coating, which makes the coating suitable for machining ductile irons and gray irons. XRD analysis was carried out on cBN powder and the cBN–TiN composite coating on WC–Co substrate. Clearly, cBN crystal phase (220) and TiN crystal phases were detected in the composite coating [22]. Analysis of the coated tool at different regions confirmed similar patterns. The presence of cBN crystal phase in the coating indicated
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1.00E-13 Attractive 1.00E-12
Repulsive
Force, N
1.00E-11
1.00E-10
1.00E-09
1.00E-08
1.00E-07 0
0.00001
0.00002
0.00003
0.00004
Coating thickness, m Fig. 1. Variation of electrostatic forces with powder coating thickness for cBN particles with average particle size of 0.6 μm and typical cluster size of 5 μm.
that infiltration of TiN at elevated temperature retains the crystal phases of cBN particles, which is critical for the coating stability and machining and wear-resistant applications. 3.4. Coating adhesion For the cBN–TiN coating, its adhesion was qualitatively assessed by using an indentation method (120° diamond indenter) at a loading of 100 kgf. The indentation mark showed a few radial cracks surrounding it, which is typical for hard coatings. Additionally, the coating adhesion was quantified by the use of a scratch test at a constant loading of 10 kgf. Scratch testing did not show any sign of particle pull-out or spalling of the coating, indicating super bonding among the cBN particles as well as between the coating and substrate. A high resolution SEM image shows that cBN particles are well bonded in the TiN matrix. Scanning transmission electron microscopy (STEM) analysis of the boundary between cBN particle and TiN matrix indicated the presence of a transition layer (less than 2 nm) chemically consisting of Ti, N, and B. A similar transition layer was also observed at the coating-to-substrate interface. This observation implies that chemical diffusion bonding dominates the adhesion between particles, coating and substrate. The enhanced bonding strength further contributes to the improved tool life. 3.5. Machining testing in continuous turning 3.5.1. Turning of AISI 4340 hardened steel Continuous turning AISI 4340 hardened steel was carried out by using the cBN–TiN composite coating coated inserts, CNMA432, and industrial benchmark inserts (CNMA432) including PCBN tipped with a
A
K-land, Al2O3 bulk inserts (with 30% of TiC) with a K-land, MT-CVD TiN–TiCN–Al2O3–TiN coated inserts, and PVD TiAlN coated inserts at identical conditions listed in Table 1. Fig. 5 shows the tool wear comparison in continuous finish turning of the steel. Evidently, the cBN–TiN composite coating coated inserts outperformed all industrial benchmark inserts used in this study based on the tool failure criteria specified in ISO 3685 for finish hard turning. Typically, machining of steel generates high temperatures. Tool failure is mainly caused by abrasive wear and diffusion wear. Examination of the cutting edges showed that the composite coating coated inserts had tool wear mainly on the flank surface at the trailing edge, which dominated the whole machining process. The tool wear was characteristic of abrasive wear and had a steady wear progression up to about 25 min, and then reached a steady wear plateau throughout the rest of the testing process. The linear progression wear period corresponds to the onset of rapid wear that is related to the pure TiN capping layer (~3 μm), and the plateau occurs after the transition from the pure TiN layer to the cBN–TiN composite layer. There was slight crater wear, but it was not significant [Fig. 6(A)]. SEM analysis of the wear plateau on the flank surface showed cBN particles (black dots) retained in the TiN matrix [Fig. 6(B)], which provided the resistance to abrasive wear required for machining steel. This observation substantiates the remarks on coating adherence discussed in Section 3.3. PCBN tipped inserts had the tool wear at the flank surface on the same side as the composite coated inserts. In addition to linear progression wear throughout the whole testing period, the PCBN inserts developed notch wear at the trailing edge [Fig. 6(C)]. Both Al2O3 bulk and MT CVD TiN–TiCN–Al2O3–TiN coated inserts had approximately the same wear rate as cBN–TiN coated inserts in the
B
cBN porous coating preform
Fig. 2. SEM images showing (A) cross-section thickness profile of cBN porous coating, and (B) surface morphology of the coating preform.
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B
A
TiN Infiltrated
TiN Infiltrated
C
D
TiCN Infiltrated
F
E
ZrCN-TiCN Infiltrated
ZrCN-TiCNInfiltrated
Fig. 3. SEM images showing (A) surface morphology of TiN infiltration, (B) cross-section of cBN–TiN, (C) surface morphology of TiCN infiltration, (D) cross-section of cBN–TiCN, (E) surface morphology of ZrCN, and (F) cross-section of cBN–ZrCN.
linear progression wear period up to about 25 min, then experienced expedited wear, implying that either the edge of the inserts was fractured or the coating reached its effective usage life, as shown in Fig. 6(D) and (E), respectively. The PVD TiAlN coated inserts had rapid wear in the whole process of machining. These inserts developed serious flank wear and crater wear as well, in addition to evident deformation on the cutting edge [Fig. 6(F)]. For all these cases with the specific conditions used in this study, wear was observed mainly on secondary flank side (trailing edge), which is consistent with the
A
observation from machining thermal spray Ni–Al alloy coating using carbide and PCBN inserts [23]. Additional tests at the exact machining parameters were carried out on 4340 hardened steel without cutting fluid. It was noticed that the effect of cutting fluid on tool performance of PCBN is about 10%, while this effect on the tool life of the cBN–TiN coated inserts is up to 25–30%. This observation implies that the cBN–TiN coated inserts work better for machining with cutting fluid if maximum tool life is one of the major goals.
B
Fig. 4. Images showing (A) a thin cBN composite coating, (B) a dense cBN composite coating.
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0.400 cBN-TiN composite coating
Tool flank wear, mm
PCBN tipped 0.300
Al2O3-TiC MT-CVD TiN-TiCN-Al2O3-TiN PVD TiAlN Coating
0.200
0.100
0.000 0.00
10.00
20.00
30.00
40.00
50.00
Machining time, min Fig. 5. A comparison of tool wear between cBN–TiN composite coating coated inserts and benchmark inserts including PCBN tipped with a K-land, Al2O3 bulk inserts (with 30% of TiC) with a K-land, MT-CVD TiN–TiCN–Al2O3–TiN coated inserts, and PVD TiAlN coated inserts in continuous turning of AISI 4340 hardened steel (Insert style: CNMA432; Vc= 150 m/min, f = 0.15 mm/rev, αp = 0.25 mm).
3.5.2. Turning of AISI 4140 prehardened steel The wear resistance of the cBN–TiN composite coating on CNMG432 inserts was also evaluated in continuous turning of AISI 4140 prehardened steel. Its performance was compared to that of MT CVD multi-
A
layer coatings (TiN–TiCN–Al2O3–TiN) on CNMG432 inserts from two different manufacturers. As shown in Fig. 7, the cBN–TiN composite produced a tool life of about 17 min, approximately 70–80% more than about 10 min for the MT CVD multi-layer coated inserts, regardless of
B
cBN-TiN coating on CNMA432 after testing
C
D
E
F
Fig. 6. SEM images showing tool wear on (A) cBN–TiN coated insert, (B) wear plateau on flank surface, (C) PCBN tipped insert, (D) Al2O3 bulk insert, (E) MT CVD multi-layer coated insert, and (F) PVD TiAlN coated insert.
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0.300
Flank wear, mm
0.250
0.200
0.150
cBN-TiN composite coating
0.100
CVD multiple-layer coating-source 1 0.050
0.000 0.00
CVD multiple-layer coating-source 2
5.00
10.00
15.00
20.00
Machining time, min Fig. 7. A comparison of tool life between cBN–TiN composite coating coated inserts and MT-CVD TiN–TiCN–Al2O3–TiN coated inserts in continuous turning of AISI 4140 pre-hardened steel (insert style: CNMG432; Vc = 183 m/min, f = 0.41 mm/rev, αp = 1.78 mm).
slightly higher initial tool wear observed on the cBN–TiN coated inserts. The high initial wear is related to the capping layer of TiN. Examination of the tested inserts indicated clearly abrasive wear on the flank surfaces both at the leading edge and trailing edge. The wear at the leading edge is more prominent than that at the trailing edge. Additionally, built-up-edge (BUE) was also observed at the leading edge. The occurrence of BUE typically worsens the tool wear due to its effects on shedding chips. 4. Conclusion The design of the composite coating for different applications was briefly discussed. Coating characterization results including surface morphology and microstructures were presented. As a study case, the cBN–TiN composite coating was characterized for its elemental composition, crystal phases, and adhesion to the substrate. Characterization results indicated a well adherent coating with uniform coating thickness and cBN particles evenly distributed in the TiN matrix. Further, the coating was tested for its machining performance in continuous turning of AISI 4340 hardened steels and AISI 4140 pre-hardened steels at representative application conditions, and compared to corresponding industrial benchmarks. Test results showed that the composite coating outperformed its industrial counterparts, polycrystalline cubic nitride, and titanium aluminum nitride by physical vapor deposition, multi-layer coating by chemical vapor deposition, and aluminum oxide bulk tools, in the respective applications. Acknowledgments The authors would like to acknowledge Krishana Narasimhan of Valenite Inc. for his help in the MT CVD ZrCN–TiCN infiltration service. The authors also would like to acknowledge Bob Fink for reviewing the manuscript, Jerry Plumlee, Justin Lowrey, and Brett McAfee for their assistance in carrying out machining tests. Arkansas Analytical
Laboratory at University of Arkansas is acknowledged for providing access to its analytical facilities.
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
A novel cBN composite coating design and machine testing: A case study in turning Wenping Jiang a, Ajay P. Malshe a, b,⁎ a b
NanoMech Inc., Springdale, AR 72764, United States Materials Research and Manufacturing Laboratories, Department of Mechanical Engineering, University of Arkansas, Fayetteville, AR 72701, United States
a r t i c l e
i n f o
Article history: Received 15 February 2011 Accepted in revised form 4 July 2011 Available online 13 July 2011 Keywords: Cubic boron nitride Composite coating Deposition Characterization Machining performance Tool wear
a b s t r a c t A composite coating series based on nano- and micro-sized cubic born nitride particulates and choice of application-specific binders were developed for turning engineering materials. The coating series were produced via two sequential processes: electrostatic spray coating of cubic boron nitride particles with size less than 2 μm for a conformal porous coating preform of designed thickness; and chemical vapor infiltration of ceramic binder phase(s) at a temperature of around 1000 °C for a dense and well adherent composite coating. In this paper, the coating design for different applications is discussed. As a study case, cubic boron nitride–titanium nitride composite coating was characterized by use of different techniques for coating cross-section, elemental composition, crystal phases, and adhesion strength. Characterization results indicated a composite coating with uniform coating thickness and evenly distributed cubic boron nitride particles in a titanium nitride matrix. Additionally, the coating was tested for its machining performance in continuous turning of AISI 4340 hardened steels and AISI 4140 pre-hardened steels at representative application conditions, and compared to corresponding industrial benchmarks. Testing results showed that the composite coating outperforms its industrial counterparts, polycrystalline cubic boron nitride compacts, titanium aluminum nitride coating by physical vapor deposition, multi-layer coating by chemical vapor deposition, and aluminum oxide bulk tools, in their respective applications. © 2011 Published by Elsevier B.V.
1. Introduction Cubic boron nitride (cBN) in bulk form and as a coating material is of significant interest especially for cutting tools due to its high hardness, thermal stability and chemical inertness up to 1200 °C in machining ferrous materials [1]. In cutting tools, cBN is mainly used as bulk or tipped form, and possesses many of the desirable properties for cutting tools. Thus, cBN is one of the favorable tool material choices for machining engineering materials such as hardened steel, pre-hardened steel, and cast irons, in spite of its high cost and limited geometry options resulting from its fabrication, which uses a high-pressure and high-temperature (HP–HT) process. Driven by the need for more options for application-specific tools such as inserts with chip breakers and round tools, and the need for higher productivity, development effort has been reported in exploring cBN coatings or thin films for cutting tool applications [2,3]. Various physical and chemical vapor deposition (PVD and CVD, respectively) techniques have been utilized for the deposition of cBN films [4–13], and been able to deposit nearly phase-pure adherent cBN films up to 2 μm for cutting tools. Keunecke et al. [2] reported a 1.2 μm
⁎ Corresponding author at: NanoMech Inc., Springdale, AR 72764, United States. E-mail address: [email protected] (A.P. Malshe). 0257-8972/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2011.07.008
thick cBN film on TiN and TiAlN pre-coated cemented carbides with a thin B-C-N gradient layer (in the range of 100–200 nm) deposited using a radio-frequency diode sputtering setup. The cBN deposited inserts demonstrated significantly extended tool life in turning H13 steel (~52 HRC) and milling alloyed tool steel. Chong et al. [14] synthesized a 2.0 μm thick cBN film on diamond pre-coated cemented carbide by using an electron cyclotron resonance microwave plasma-chemical vapor deposition process. The superior chemical resistance and extreme hardness from the combination of diamond transitional layer and cBN film may have potential for machining steels and ferrous materials. Clearly, these techniques have demonstrated the potential to provide economic alternatives to the high temperature/high pressure synthesis techniques. However, issues including (1) high compressive stress (in the order of a few GPa up to 20 GPa) existing unfavorably at the substrate–coating interface and leading to the delamination of the coating from the substrate [15], (2) low deposition rate [11], and (3) existence of hexagonal boron nitride (h-BN) and/or turbostratic boron nitride (t-BN) in the synthesized film [15], where the non-cubic phases are soft and contain defects such as boron dangling bonds, leading to mechanical destabilization of the coating or film, are still the major hindrances associated with the growth of cBN films in desirable thicknesses for machining applications. An advancement in synthesizing thick (5–20 μm) cBN based composite coatings has been made for machining applications
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[16–18]. It starts with electrostatic spray coating (ESC) of presynthesized cBN crystals as a porous powder coating preform, followed by chemical vapor infiltration (CVI) of ceramic phases such as TiN, TiCN, etc. as a binder to form an adherent composite coating. In the work reported here, the coating design needed for different applications was discussed. As a case study, a cubic boron nitride– titanium nitride composite coating was characterized by use of different techniques for examining the coating cross-section, elemental composition, crystal phases, and adhesion strength. Further, machining test results in continuous turning of AISI 4340 hardened steel and AISI 4140 pre-hardened steel were presented and compared to performance data from industrial benchmarks.
2. Experimental 2.1. Coating deposition process The coating was synthesized by a two-step sequential process starting with the deposition of pre-synthesized cBN powder by using ESC to form a porous coating preform of desired thickness and pore size, followed by CVI of ceramic phases as a binder for achieving an adherent coating to the substrate. The ESC deposited cBN powder coating preform loosely adheres to the substrate by electrostatic forces, and largely controls the coating surface smoothness or morphology [19], while CVI is critical to ensure fully dense coating. For a tool surface with a porous cBN particulate layer, infiltration should begin at the substrate surface, and proceed toward the top of the coating preform, instead of the other directions because, in the infiltration process, formation of dense film may lead to incomplete diffusion of the ceramic binder, leaving behind unfilled pores that are detrimental to coating performance. Thus, it is necessary to control the infiltration process in such a way that the diffusion rate of gaseous phases is higher than the reaction rate. The details of the coating process can be found elsewhere [16–18].
2.2. Characterization of the coating The coating thickness was measured by using a standard ball crater tester, which rotates a carbide ball (diameter of 20 mm) with a paste of 0.1 μm nanocrystalline diamond particles, and was confirmed by optical characterization of the cross-section of the coating. In ball crater testing, the ball rotating against the specimen surface creates a circular wear track as deep as the coating thickness. The radius of the track is then measured by using optical microscopy. Thus, the thickness of the coating can be derived by following the related geometrical equation. cBN particle distribution in the composite coating was characterized using scanning electron microscopy (SEM, XL30, Phillips). The surface roughness (Ra) of the coating was measured with a surface profilometer (Veeco Dektak 3030). X-ray diffraction equipment (XRD, Philips X'Pert dual goniometer diffractometer) was used to analyze the material phases in the coating. For comparison, XRD analysis was also applied to the aspurchased cBN powder at the same XRD settings. The adhesion of the coating to the substrate was evaluated by the use of the Rockwell C indentation method on a conventional hardness tester (ADR-12). The load applied on the indenter was 100 kgf. The adhesion between the coating and the substrate was assessed based on observation of the radial and peripheral cracks. Additionally, a scratch test was performed by running a Rockwell C diamond indenter (120° cone with 200 μm radius hemispherical tip) across the crater created by the ball crater tester and followed by optical observation of the scratch to quantify critical load. These analyses provide useful insight into the behavior of the coating for turning AISI 4340 hardened steel and AISI 4140 pre-hardened steel.
2.3. Machining tests Wear resistance of the composite coating was evaluated by conducting continuous turning tests on AISI 4340 hardened steel and AISI 4140 pre-hardened steel at typical machining conditions recommended by tool manufacturers for each specific material. In turning AISI 4340 hardened steel, CNMA432 inserts were used, while in turning AISI 4140 pre-hardened steel, CNMG432 inserts were used. Table 1 lists the details of workpiece materials, machining conditions, benchmark inserts, and tool failure criteria used in the machining tests. In the testing process, tools were inspected under a toolmakers microscope at regular intervals for their wear developed at different locations, which include flank surface, nose, and rake surface. The tool life is defined as the time corresponding to the given criterion (Table 1). The tool performance of the composite coating coated inserts was compared to that of the industrial benchmark tools at identical conditions. The tested inserts were further analyzed by using optical microscope and SEM for their failure modes. 3. Results and discussion 3.1. Coating design for machining of different materials A general requirement for coatings on cutting tools is to provide the desirable properties, which include (A) strong adhesion to tool substrate, (B) high hardness and hot hardness, (C) strong abrasive wear resistance, (D) high facture toughness; (E) high chemical stability in contact with workpiece material at high contact pressure and elevated temperature; and (F) low thermal conductivity and friction coefficient [20]. Depending on specific workpiece material and machining conditions, the above desirable properties can be achieved by tailoring coating thickness, different binders, and adjusting volumetric ratio of cBN in the ceramic binder matrix. In reference to the typical applications of PCBN tools, and the recommended applications for PVD and CVD mono-layer and multiple-layer coatings, a snapshot of the composite coating design is listed in Table 2. For a specified application, an effective solution is the combination of substrate, edge preparation, and suitable coating. Especially, good adhesion between the coating and substrate is critical. Thus, matching of thermal expansion coefficient (TEC) between the coating material and substrate, in addition to chemical compatibility, is preferable especially for CVI at a temperature approaching 1000 °C. Otherwise, an interfacial layer or compositionally graded layer is needed to minimize possible stress build-up, which is considered one of the major causes for coating delamination. In this case, cBN has a TEC in the vicinity of 5–6 × 10− 6/K; TiN has a TEC of 9.4× 10− 6/K. If 45% (volumetric) of cBN particles is in the TiN matrix, then the TEC of the composite is estimated as 7.5 × 10− 6/K by taking the respective volumetric ratio as a weighing factor. Therefore, a generic C2 grade is preferred because it has a TEC of 5–7 × 10 − 6/K, close to that of cBN–TiN. If TiCN (TEC = 8.4 × 10− 6/K) is used as an infiltrant with the same cBN ratio as cBN in TiN, then the TEC of the composite (cBN–TiCN) will be around 6.8 × 10 − 6/K, which is even closer to the TEC of WC–Co. Based on the above consideration, for general continuous turning applications, a coating thickness of 10–15 μm was selected to meet adhesion and other general application requirements. The selected generic C2 grade (~6.0%) has micron size WC grains for balanced hardness and toughness. The edge of the insert was just de-burred without honing. While for machining with interruptions such as oil holes or key ways present on workpieces, the selection of substrate, edge preparation, and infiltrants will be different from those recommended for continuous machining. In this case, toughness of the combination of coating and substrate is critical, in addition to hardness for wear resistance. Therefore, substrates with high Co percentage (typical 7.5% or above is preferred) and edge preparation is needed. The edge hone varies
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Table 1 Workpiece materials, machining conditions, benchmark inserts, and tool failure criterion used in machining tests. Workpiece materials
Machining conditions
Benchmark inserts
AISI 4340 hardened steel; hardness: 50–52 HRC; D = 62.5 mm (2.5"), L = 254 mm (10.0")
Vc = 150 mm/min f = 0.15 mm/rev αp = 0.25 mm with cutting fluid
ISO 3685, flank wear of 0.2 mm (0.008") A) Polycrystalline cubic boron nitride or surface finish significant degradation (PCBN with CBN of 55% in volume, CNMA432) B) PVD TiAlN coated CNMA432 (~4 μm) C) CVD TiCN–Al2O3–TiN coated CNMA432 (~15 μm); D) Bulk Al2O3 CNMA432 (with 30% of TiC) CVD TiCN–Al2O3–TiN coated CNMG432 (~15 μm) ISO 3685, flank wear of 0.2 mm (0.008")
AISI 4140 pre-hardened steel; hardness: 25–32 HRC; Vc = 183 m/min; D = 62.5 mm (2.5"), L = 254 mm (10.0") f = 0.41 mm/rev αp = 1.78 mm with cutting fluid
according to degree of interruptions. For severe interruptions, slightly large hone (N0.025 mm) is chosen to avoid edge chip-off. In terms of coating, combination of a specific density of cBN with different infiltrants such as TiCN and Al2O3 is suggested. Overall coating thickness is typically limited to 10 μm. Machining tests on 4340 hardened steel bars with a key way of 9.5 mm (3/8") in longitudinal direction using the composite coating coated CNMA432 inserts of high Co percentage have demonstrated tool life being superior to that of similar inserts with CVD multiple-layer coating (with Al2O3) on Co enriched carbide from major tool manufacturers. Details of the coating and machining tests with interruptions will be presented in the future.
3.2. Porous coating formation by electrostatic spray coating With a given average particle size of cBN powder (b2.0 μm), the deposition of cBN particles using ESC is affected by the physical properties (particle density, particle shape, particle size distribution, and electrical resistivity) of cBN powder, in addition to process parameters including electrode–substrate distance, electrical voltage applied at the electrode, and main air pressure. In the ESC process, aerodynamic forces and electrostatic forces (both attractive and repulsive forces) largely govern the deposition of the charged particles. In regions relatively far away from the grounded substrate, aerodynamic force dominates; electrostatic forces play an influential role in regions close to the substrate. As the cBN powder layer is built up, it starts to form a mirror electrostatic field to deter the coming of more charged particles. Thus, the formation of a porous coating preform is the result of attractive force and repulsive force. The balance of these interacting forces determines the maximum possible thickness of a coating preform for a given powder material at a specified deposition condition. Based on the given cBN particles with average particle size of 0.6 μm and typical cluster size of 5 μm, and an optimized condition (voltage: −60 kV; substrate–electrode distance: 150 mm) [21], the coating preform thickness could be as thick as 14 μm, as depicted in Fig. 1. The details of the modeling process for the attractive force and repulsive force for charged cBN particles can be found in [19]. A typical cross-section thickness profile and the surface morphology of the coating preform are shown in Fig. 2(A) and (B), respectively.
Table 2 A snapshot of cBN based composite coating design for different engineering materials. Infiltrants Coating form
Potential applications
TiN TiCN TiCN– Al2O3 HfN
Low carbon steel, alloy steel (partially), and nodular cast iron Alloy steel, ductile iron, cast iron, and stainless steel Hardened steel, tool steels, stainless steel, titanium alloy (partially), and Ni-based super alloy Hardened steel, and all other above materials
cBN–TiN cBN–TiCN cBN– TiCN/Al2O3 cBN–HfN
Tool failure criteria
3.3. Coating structure Fig. 3 illustrates the surface morphology and cross-section of the cBN based composite coating with TiN, TiCN, and ZrCN infiltrants, respectively. For the TiN infiltration, the surface has relatively fine grain size with features showing facets of cubic structures [Fig. 3(A)]; the cross-section clearly indicates two distinct layers, a cBN particulate based composite layer (~13 μm) with particles in a reasonably uniform distribution across the TiN matrix and a pure TiN layer (about 3 μm) on top of the composite layer [Fig. 3(B)]. The thickness is consistent with the average thickness, 15± 1.0 μm, obtained from ball crater testing. The cBN particle density in this case is about 40–45%, but can be tailored by slightly changing the particle deposition process. The thickness of the pure TiN layer can be adjusted by modifying the process time in the infiltration process. Elemental composition analysis by using EDS on the coating surface indicated a Ti/N ratio of 0.89. This ratio was reduced to 0.52 in the composite coating due to the contribution of N from cBN. For TiCN infiltration, the surface shows similar morphology as that of TiN infiltration, but with relatively coarse grain [Fig. 3(C)] due to the fairly high temperature (N1030 °C) associated with the infiltration process. The cBN particles are well “locked” in the TiCN matrix, as shown in Fig. 3(D). TiCN has a thermal expansion coefficient closer to cBN than TiN, thus, the tendency for crack initiation is less than with TiN. Fig. 3(E) illustrates the surface morphology of a ZrCN infiltrated composite coating, which demonstrates nodules consisting of many fine needle-shaped crystals. The nodules are more or less related to the cBN particle clusters or agglomerates, while lenticules are from the ZrCN. A high magnification SEM image shows “mushy” zones around the cBN particles, which are integrated to the ZrCN matrix for adhesion [Fig. 3(F)]. The surface roughness (Ra) of the coating with different infiltrants is in the range of 3–5 μm. Depending on the requirements for different applications, the composite coating can be tailored to as thin as a few microns with relatively high density of cBN particles. Additionally, the density of cBN particles can be adjusted with different particle volumetric ratio. Fig. 4 illustrates the sample cross-sections of thin and high cBN density composite coatings, respectively. As shown in Fig. 4(A), the overall coating thickness (including capping layer) is about 4 μm with cBN–TiN composite layer of about 1.0 μm. Capping layer, TiN, stacked on the top of cBN–TiN forms a unique layered structure, which is important for machining specifically with interruptions. Fig. 4(B) is a cross section of the composite coating with up to 70% (volumetric) cBN particles. The high cBN density significantly enhances the wear resistance of the coating and modifies the thermal conductivity of the composite coating, which makes the coating suitable for machining ductile irons and gray irons. XRD analysis was carried out on cBN powder and the cBN–TiN composite coating on WC–Co substrate. Clearly, cBN crystal phase (220) and TiN crystal phases were detected in the composite coating [22]. Analysis of the coated tool at different regions confirmed similar patterns. The presence of cBN crystal phase in the coating indicated
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1.00E-13 Attractive 1.00E-12
Repulsive
Force, N
1.00E-11
1.00E-10
1.00E-09
1.00E-08
1.00E-07 0
0.00001
0.00002
0.00003
0.00004
Coating thickness, m Fig. 1. Variation of electrostatic forces with powder coating thickness for cBN particles with average particle size of 0.6 μm and typical cluster size of 5 μm.
that infiltration of TiN at elevated temperature retains the crystal phases of cBN particles, which is critical for the coating stability and machining and wear-resistant applications. 3.4. Coating adhesion For the cBN–TiN coating, its adhesion was qualitatively assessed by using an indentation method (120° diamond indenter) at a loading of 100 kgf. The indentation mark showed a few radial cracks surrounding it, which is typical for hard coatings. Additionally, the coating adhesion was quantified by the use of a scratch test at a constant loading of 10 kgf. Scratch testing did not show any sign of particle pull-out or spalling of the coating, indicating super bonding among the cBN particles as well as between the coating and substrate. A high resolution SEM image shows that cBN particles are well bonded in the TiN matrix. Scanning transmission electron microscopy (STEM) analysis of the boundary between cBN particle and TiN matrix indicated the presence of a transition layer (less than 2 nm) chemically consisting of Ti, N, and B. A similar transition layer was also observed at the coating-to-substrate interface. This observation implies that chemical diffusion bonding dominates the adhesion between particles, coating and substrate. The enhanced bonding strength further contributes to the improved tool life. 3.5. Machining testing in continuous turning 3.5.1. Turning of AISI 4340 hardened steel Continuous turning AISI 4340 hardened steel was carried out by using the cBN–TiN composite coating coated inserts, CNMA432, and industrial benchmark inserts (CNMA432) including PCBN tipped with a
A
K-land, Al2O3 bulk inserts (with 30% of TiC) with a K-land, MT-CVD TiN–TiCN–Al2O3–TiN coated inserts, and PVD TiAlN coated inserts at identical conditions listed in Table 1. Fig. 5 shows the tool wear comparison in continuous finish turning of the steel. Evidently, the cBN–TiN composite coating coated inserts outperformed all industrial benchmark inserts used in this study based on the tool failure criteria specified in ISO 3685 for finish hard turning. Typically, machining of steel generates high temperatures. Tool failure is mainly caused by abrasive wear and diffusion wear. Examination of the cutting edges showed that the composite coating coated inserts had tool wear mainly on the flank surface at the trailing edge, which dominated the whole machining process. The tool wear was characteristic of abrasive wear and had a steady wear progression up to about 25 min, and then reached a steady wear plateau throughout the rest of the testing process. The linear progression wear period corresponds to the onset of rapid wear that is related to the pure TiN capping layer (~3 μm), and the plateau occurs after the transition from the pure TiN layer to the cBN–TiN composite layer. There was slight crater wear, but it was not significant [Fig. 6(A)]. SEM analysis of the wear plateau on the flank surface showed cBN particles (black dots) retained in the TiN matrix [Fig. 6(B)], which provided the resistance to abrasive wear required for machining steel. This observation substantiates the remarks on coating adherence discussed in Section 3.3. PCBN tipped inserts had the tool wear at the flank surface on the same side as the composite coated inserts. In addition to linear progression wear throughout the whole testing period, the PCBN inserts developed notch wear at the trailing edge [Fig. 6(C)]. Both Al2O3 bulk and MT CVD TiN–TiCN–Al2O3–TiN coated inserts had approximately the same wear rate as cBN–TiN coated inserts in the
B
cBN porous coating preform
Fig. 2. SEM images showing (A) cross-section thickness profile of cBN porous coating, and (B) surface morphology of the coating preform.
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B
A
TiN Infiltrated
TiN Infiltrated
C
D
TiCN Infiltrated
F
E
ZrCN-TiCN Infiltrated
ZrCN-TiCNInfiltrated
Fig. 3. SEM images showing (A) surface morphology of TiN infiltration, (B) cross-section of cBN–TiN, (C) surface morphology of TiCN infiltration, (D) cross-section of cBN–TiCN, (E) surface morphology of ZrCN, and (F) cross-section of cBN–ZrCN.
linear progression wear period up to about 25 min, then experienced expedited wear, implying that either the edge of the inserts was fractured or the coating reached its effective usage life, as shown in Fig. 6(D) and (E), respectively. The PVD TiAlN coated inserts had rapid wear in the whole process of machining. These inserts developed serious flank wear and crater wear as well, in addition to evident deformation on the cutting edge [Fig. 6(F)]. For all these cases with the specific conditions used in this study, wear was observed mainly on secondary flank side (trailing edge), which is consistent with the
A
observation from machining thermal spray Ni–Al alloy coating using carbide and PCBN inserts [23]. Additional tests at the exact machining parameters were carried out on 4340 hardened steel without cutting fluid. It was noticed that the effect of cutting fluid on tool performance of PCBN is about 10%, while this effect on the tool life of the cBN–TiN coated inserts is up to 25–30%. This observation implies that the cBN–TiN coated inserts work better for machining with cutting fluid if maximum tool life is one of the major goals.
B
Fig. 4. Images showing (A) a thin cBN composite coating, (B) a dense cBN composite coating.
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0.400 cBN-TiN composite coating
Tool flank wear, mm
PCBN tipped 0.300
Al2O3-TiC MT-CVD TiN-TiCN-Al2O3-TiN PVD TiAlN Coating
0.200
0.100
0.000 0.00
10.00
20.00
30.00
40.00
50.00
Machining time, min Fig. 5. A comparison of tool wear between cBN–TiN composite coating coated inserts and benchmark inserts including PCBN tipped with a K-land, Al2O3 bulk inserts (with 30% of TiC) with a K-land, MT-CVD TiN–TiCN–Al2O3–TiN coated inserts, and PVD TiAlN coated inserts in continuous turning of AISI 4340 hardened steel (Insert style: CNMA432; Vc= 150 m/min, f = 0.15 mm/rev, αp = 0.25 mm).
3.5.2. Turning of AISI 4140 prehardened steel The wear resistance of the cBN–TiN composite coating on CNMG432 inserts was also evaluated in continuous turning of AISI 4140 prehardened steel. Its performance was compared to that of MT CVD multi-
A
layer coatings (TiN–TiCN–Al2O3–TiN) on CNMG432 inserts from two different manufacturers. As shown in Fig. 7, the cBN–TiN composite produced a tool life of about 17 min, approximately 70–80% more than about 10 min for the MT CVD multi-layer coated inserts, regardless of
B
cBN-TiN coating on CNMA432 after testing
C
D
E
F
Fig. 6. SEM images showing tool wear on (A) cBN–TiN coated insert, (B) wear plateau on flank surface, (C) PCBN tipped insert, (D) Al2O3 bulk insert, (E) MT CVD multi-layer coated insert, and (F) PVD TiAlN coated insert.
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0.300
Flank wear, mm
0.250
0.200
0.150
cBN-TiN composite coating
0.100
CVD multiple-layer coating-source 1 0.050
0.000 0.00
CVD multiple-layer coating-source 2
5.00
10.00
15.00
20.00
Machining time, min Fig. 7. A comparison of tool life between cBN–TiN composite coating coated inserts and MT-CVD TiN–TiCN–Al2O3–TiN coated inserts in continuous turning of AISI 4140 pre-hardened steel (insert style: CNMG432; Vc = 183 m/min, f = 0.41 mm/rev, αp = 1.78 mm).
slightly higher initial tool wear observed on the cBN–TiN coated inserts. The high initial wear is related to the capping layer of TiN. Examination of the tested inserts indicated clearly abrasive wear on the flank surfaces both at the leading edge and trailing edge. The wear at the leading edge is more prominent than that at the trailing edge. Additionally, built-up-edge (BUE) was also observed at the leading edge. The occurrence of BUE typically worsens the tool wear due to its effects on shedding chips. 4. Conclusion The design of the composite coating for different applications was briefly discussed. Coating characterization results including surface morphology and microstructures were presented. As a study case, the cBN–TiN composite coating was characterized for its elemental composition, crystal phases, and adhesion to the substrate. Characterization results indicated a well adherent coating with uniform coating thickness and cBN particles evenly distributed in the TiN matrix. Further, the coating was tested for its machining performance in continuous turning of AISI 4340 hardened steels and AISI 4140 pre-hardened steels at representative application conditions, and compared to corresponding industrial benchmarks. Test results showed that the composite coating outperformed its industrial counterparts, polycrystalline cubic nitride, and titanium aluminum nitride by physical vapor deposition, multi-layer coating by chemical vapor deposition, and aluminum oxide bulk tools, in the respective applications. Acknowledgments The authors would like to acknowledge Krishana Narasimhan of Valenite Inc. for his help in the MT CVD ZrCN–TiCN infiltration service. The authors also would like to acknowledge Bob Fink for reviewing the manuscript, Jerry Plumlee, Justin Lowrey, and Brett McAfee for their assistance in carrying out machining tests. Arkansas Analytical
Laboratory at University of Arkansas is acknowledged for providing access to its analytical facilities.
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