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Surface & Coatings Technologyjournal homepage:www.elsevier.com/locate/surfcoat
Surface modification of M2 steel by combination of cathodic cage plasma
deposition and magnetron sputtered MoS
-TiN multilayer coatings
, G.B. Praxedesb
, L.L.F. Limab
, I.G. Nascimentoa
, R.R.M. Sousac
, M. Naeemd,⁎
, S.M. Alvesa
, Javed Iqbale
aPrograma de Pós- Graduação em Engenharia Mecânica – UFRN, Brazil bEscola de Ciência e Tecnologia (ECT) – UFRN, Brazil
cDepartamento de Engenharia de Materiais – UFPI, Brazil
dDepartment of Physics, Women University of Azad Jammu and Kashmir, Bagh, Pakistan eDepartment of Physics, University of Azad Jammu and Kashmir, Muzaffarabad, Pakistan
A R T I C L E I N F O Keywords:
HSS M2 steel Titanium nitride coating Molybdenum disulfide coating Solid lubrication
Indentation and hardness
A B S T R A C T
Titanium nitride (TiN) is a good choice for the improvement in surface hardness of high-speed steel. Unfortunately, it has low adhesion with substrate and exhibits high friction coefficient; as a result it does not provide sufficient protection against sliding wear in metal-to-metal contact. The adhesion problem can be re-moved by nitriding process, whereas friction coefficient can be reduced by solid lubrication coating. In this study, an attempt is made to synthesize TiN hard coating as well as solid lubrication coating of molybdenum disulfide (MoS2) using magnetron sputtering, along with substrate pre-treatment by cathodic cage plasma
de-position using titanium cathodic cage. The cathodic cage plasma nitrided sample exhibits significantly higher surface hardness, which reduced by solid lubrication coating. The nitrided sample depicts the presence of iron nitrides, TiN and nitrogen diffused martensite phases, whereas coated samples shows the presence of MoS2and
TiN phases. The friction coefficient and machining temperature are dramatically reduced by lubrication coating. This study recommends that the use of cathodic cage plasma nitriding using titanium cathodic cage is beneficial for improved surface hardness, and addition of solid lubrication coating is beneficial for reducing the coefficient of friction and machining temperature by scarifying hardness. As, both the systems are already proven to be appropriate for industrial-scale uses, thus results from this study can be applied for industrial-scale application.
The lifetime of high-speed steel (HSS) cutting tool is of noteworthy importance in surface engineering applications . In order to enhance the performance of tools in machining applications such as machining speed, useful life time and quality of obtained product, variety of in-dustrial coatings are under consideration . Although, the cost of tools is usually not so high, but it significantly affects the total ma-chining cost: in order to attain the high mama-chining speed (i.e. to in-crease the productivity), the life time of tool must be enhanced, which reduces the maintenance and personnel cost. The machining perfor-mance of tools can be enhanced by several coatings (such as titanium nitride) by physical vapor deposition, which is beneficial for mechan-ical and tribologmechan-ical aspects [2,3]. Unfortunately, sometimes these thin films are not adequate to fulfill the industrial requirements, and ex-hibits several problems including : these coatings are quite hard and
brittle and fail rapidly from relatively rough surfaces. Due to their high friction coefficient, they do not provide sufficient protection against sliding wear in case of metal-to-metal contact. The removal of coatings in sliding wear form hard particles on the wear track, which acts as grit, and thus deteriorate the lifetime. In order to eliminate these short-comings, use of lubricants is valuable, which can reduce the friction coefficients and thus failure to coatings can be minimized.
The use of liquid lubricants is limited due to environmental hazards, high cost and operational restrictions such as in aerospace applications [5,6]. Also, liquid lubricants are not suitable for certain applications as textile industries and food processing, where direct contact with lu-bricants contaminates them and thus causes health issues . Thus the solid lubricants are preferable in several applications, which can be deposited on the surface of a material as inter-layer or coating. Among several solid lubricants, molybdenum disulfide is of substantial sig-nificance, which exhibits individual atomic-thin planes, thus the sliding
Received 2 November 2019; Received in revised form 20 December 2019; Accepted 29 December 2019
E-mail address:email@example.com(M. Naeem).
Available online 07 January 2020
0257-8972/ © 2020 Elsevier B.V. All rights reserved.
of planes against each other is quite easier and acts as a good solid lubricant . Advantageously, this lubricant can work in oxygen defi-cient-atmosphere, and thus attractive in space/aerospace environment. Beside several advantages of solid lubrication of MoS2coating, they
exhibit poor surface hardness, higher wear rate and high probability of oxidation in humidity. Thus, individually MoS2coating is not adequate
to fulfill the requirements in surface engineering applications . Therefore, usually solid lubricants are used with metal doping, or in-sertion of solid lubricant in hard matrix or a mixture of lubricant coating and hard coating.
Several authors used the MoS2 lubricant coating along with
tita-nium/titanium nitride in the form of composite coatings assisted with numerous techniques. Bae et al.  used chemical vapor deposition (CVD) technique to develop composite coating on graphite and Ti-6AI-4V substrates and found a decrease in friction coefficient. Gilmore et al.  also developed composite coating of TiN − MoS2 by using dc
magnetron sputtering, and reported the simultaneous presence of good hardness and low friction coefficient. Renevier et al. [9,10] evaluated the performance of titanium-containing MoS2composited coating
de-posited by dc magnetron sputtering, and found that tool performance and surface finishing can be enhanced by this composite coating. Jing et al.  reported the effect of Ti or TiN co-deposition on the per-formance analysis of lubricating MoS2coating by unbalanced plasma
plating technique, and found significant increase in wear resistance of compound coating. Haider et al.  deposited hard-lubricant coating by a combination of TiN and MoS2, using closed-field magnetron
sputtering. The compound coating showed a decrease in friction coef-ficient as well as hardness over only TiN, whereas as higher than MoS2
coating. Rehman et al.  reported the effect of Ti − TiN graded in-terlayer on the magnetron sputtered TiN − MoS2hard-lubricant coating
by closed-field magnetron sputtering, and found enhanced tribological features in the presence of interlayer. Piazzone et al.  synthesized TiN films with embedded MoS2inorganic fullerenes assisted with
su-personic cluster beam and cathodic arc reactive evaporation, and found improvement in nanotribological properties. Ding et al.  studied the tribological properties of chromium and titanium doped MoS2coating
deposited by unbalanced magnetron sputtering under various humidity atmosphere. They found that excellent tribological properties can be attained by using chromium and titanium doping even in the presence of humidity. Gangopadhyay et al.  reported the effect of substrate bias on the deposition of composite coating of TiN − MoSxby pulsed dc
magnetron sputtering, and found excellent wear resistance. Besides the composite coatings described above, several authors reported the ben-eficial effects of separate TiNand MoS2layers, where TiN acts as harder
coating and MoS2 as solid lubricating coating. Goller et al. 
de-veloped TiN by arc-evaporation and MoS2 by magnetron sputtering
process, and reported the formation of compact coating with better adhesion and hardness. Xu et al.  used ion-plating technique for TiN coating and magnetron sputtering for MoS2coating, and excellent
tri-bological properties were attained by duplex treatment over individual treatments. Ma et al.  reported TiNcoating and lubricating coating of MoS2, both by using unbalanced magnetron sputtering, and a
re-duction in friction was observed using duplex coating. Dhere et al.  synthesized the TiN coating by reactive dc magnetron sputtering and MoS2coating by radio-frequency (RF) magnetron sputtering, and
ob-tained dense well adherence coatings with better hardness and lu-bricating performance. Ahmed et al.  developed TiN coating by RF-magnetron sputtering and MoS2coating by vacuum thermal
evapora-tion technique, and found that dual layer exhibits low fricevapora-tion coeffi-cient with average hardness. Paskvale et al.  reported that the addition of MoS2nanotubes over the hard coatings (TiN,CrN,TiAlN) on
AISI-D2 steel reduces the wear rate by 10 times and friction coefficient by 3 times. On the other hand, the use of MoS2platelets instead of
nanotubes on the wear and friction coefficient was limited.
Above literature review clearly illustrates the beneficial effects of lubricating coating MoS2in term of low friction coefficient, and TiN
coating in term of better hardness. Thus, such duplex treatment is of countless attraction in the industrial application. However, in most of the previous work, the TiN hard coatings were developed by physical vapor deposition (arc and magnetron sputtering). Unfortunately, the titanium nitride coatings by PVD exhibit certain drawbacks: due to low-deposition efficiency, thick layer development (for wear resistance, formation of sufficiently thick layer is essential) requires long proces-sing time and thus increased total procesproces-sing cost . Usually due to difference in crystal structure of TiN with substrate, PVD-TiN coatings exhibit poor adhesion . In this study, we aimed to use additional treatment of plasma nitriding prior to the deposition of lubrication and hard coatings by magnetron sputtering. Furthermore, usually the combination of titanium nitride and molybdenum disulfide coatings was carried out in such a way that the outer layer was of lubrication coating. However, such order of treatment reduces the surface hardness drastically due to the low hardness of MoS2at the top of the surface.
Thus, we change this processing order with the aim to attain the si-multaneous combination of improved lubrication performance as well as sufficient surface hardness.
Around 20 years back, an innovative nitriding technique equipped with a cathodic cage, known as cathodic cage plasma nitriding was presented, in which samples were placed on floating potential/cathodic potential and enclosed in metal screen at cathodic potential . Ad-vantageously, this technique is based on the transformation of cathodic cage material on the sample in the form of compound after reacting with processing gasses. Due to this uniqueness, this technique is widely used for alloying sample with cathodic cage material [25,26], in ad-dition to thermochemical diffusion. Thus it is predictable that due to simultaneous presence of deposition and diffusion in cathodic cage plasma deposition (CCPD), good adhesion of TiN coating with substrate as well as with lubricating MoS2coating. The low processing cost and
high deposition efficiency of CCPD further motivate to use cathodic cage plasma TiN deposition. In a previous report, we successfully syn-thesized individual titanium nitride coating by cathodic cage plasma deposition technique on tool steel by using titanium cathodic cage . Here, in this study we tried to deposit the titanium nitride as well as nitrogen diffusion using CCPD, and later on effect of multilayers of TiN and MoS2using magnetron sputtering is investigated.
2. Experimental details
The M2 steel samples with dimension of 10 mm diameter and 3 mm thickness were used in this study (composition given inTable 1). They went through a preparation process with 220 to 2000 of granulometry and finally were polished with chemical solution (60% H2O and 40%
silica). In the hardness test, the digital microdurometer model HVS 1000aat load of 0.98 N with coupled optical microscope was used. In
order to reduce the error in measurement, average of five indentations was performed. Plasma nitriding was performed in the nitriding reactor with cathodic cage adaptation as shown inFig. 1, and details given previous report . In the multilayer film deposition stage, a mag-netron sputtering system was used as shown inFig. 2. The apparatus consists of a cylindrical borosilicate chamber with dimensions of 300 mm in diameter and 40 mm in height. The lower part of the chamber is connected to the vacuum system consisting of a mechanical pump in series with a diffusion pump. Molybdenum disulfide (MoS2)
and titanium (Ti) targets were positioned 7 cm above the sample holder (coupled in the center of the upper flange). Prior to the beginning of each treatment, the targets were subjected to 5 minute pre-sputtering to Table 1
Elemental composition of M2 steel.
Elements C Cr Mo W V Fe
remove any remaining impurities from their surface. The processing conditions using in cathodic cage plasma deposition and magnetron sputtering coatings are given inTable 2. The labeling of samples used in study is given inTable 3. The schematic illustration of cathodic cage plasma deposition and multilayers coatings by magnetron sputtering (MS) is depicted inFig. 3.
For X-ray diffraction analysis, the high-resolution Shimadzu brand diffractometer (model XRD-7000) with copper Kα radiation and Bragg-bretan configuration at 40 KV and 30 mA was used. XRD analysis was done with a 3° grazing incidence, a 0.02° pitch every 0.5 s and a scan between 30° and 80°. For morphology analysis, a Shimadzu atomic force microscope (model SPM-9700) was used, by the Modular force method with constant force and 1 Hz frequency. In this test, we used monolithic silicon tip with polygonal base pyramidal geometry, with a resonant frequency of 75 KHz. Raman scattering was performed with the LabRAM HR Evolution (HORIBA Scientific) system with 16 mW and acquisition time of 15 s, accumulation of 10 measurements with spec-tral range of 200–1200 cm−1. To evaluate surface roughness, a
Taylor-Hobson portable roughness meter surtrônic 25 was used, following ISO 4287(R). To perform the measurements, the probe was positioned perpendicular to the surface of the samples. In each sample three roughness scans were made. The cut-off used was 0.2 mm, Ln = 1 mm operation with Gaussian filter. The friction coefficients were evaluated by tribological test of the alternating motion brobe under high fre-quency, the HFRR (High Frequency Reciprocating Rig) of the PCS in-struments® with a ball with a diameter of 6 ± 0.05 mm, HV0.2
631 ± 47 of hardness AISI 52100. This test was performed with an oscillation frequency of 20 ± 1 Hz, a stroke length of 1 ± 0.02 mm, a load of 200 ± 1 gf and a temperature of 35 °C for 20 min. To measure the thickness of the films and observe the arrangement of the TiN and
Fig. 1. Schematic diagram of cathodic cage plasma deposition system.
Fig. 2. Schematic diagram of magnetron sputtering used for thin film deposition. Table 2
The processing conditions used in cathodic cage plasma deposition (CCPD) and magnetron sputtering coatings.
parameters Cathodic cage plasmadeposition Magnetron sputtering MoS2 TiN Gasses mixture 80 N2–20 H2(%) Ar (10 sccm) Ar (10 sccm)+ N2(2 sccm) Pressure (mbar) 2 4 × 10−3 4 × 10−3 Temperature °C 430 85 130 Time (h) 4 1 1.5 Current (A) 0.4 0.12 0.15 Power (W) 208 96 120 Table 3
The sample labeling by combination of cathodic cage plasma deposition (CCPD) and magnetron sputtering coatings.
Sample Labeling Cathodic cage plasma deposition Magnetron sputtering MoS2 TiN
Base – – –
A TiN-deposition/N-diffusion – – B TiN-deposition/N-diffusion 1 layer 1 layer C TiN-deposition/N-diffusion 2 layers 2 layers D TiN-deposition/N-diffusion 4 layers 4 layers E TiN-deposition/N-diffusion 6 layers 6 layers
MoS2layers, the ZEISS AURIGA 40 MEV-FEG equipment with coupled
EDS was used to identify the elemental chemical composition of the films deposited by magnetron sputtering. In order to analyze the layers thickness and check uniformity of coatings, glass samples were placed together with steel samples during the magnetron sputtering coating. Thus, the cross-sectional images, elemental mappings and elemental line scan presented here are of magnetron sputtered coatings on glass substrate. The machining test was performed with a lathe mark Nardini Model Mascote-MS205. In the facing process the following parameters were used: cutting depth (ap) 1 mm, feed (fn) 0.2 mm/r, and rotational
frequency 400 rpm providing a minimum cutting speed of 75 m/min (innermost cylinder region) and a maximum cutting speed of 126 m/ min (outmost region of the cylinder), without the use of cutting fluids. A digital infrared thermometer INSTRUTHERM (Ti-550) was used to monitor the temperature evolution in the cutting region of the tools. The friction coefficients were evaluated by tribological test of the al-ternating motion probe under high frequency, according to ASTM D6079-2018, without the use of liquid lubricant, since it is intended to analyze the lubricating effect of the MoS2layer. The high frequency
reciprocating rig (HFRR) of the PCS instruments® with a ball (AISI 52100) of diameter of 6 ± 0.05 mm and hardness HV0.2631 ± 47 was
used. This test was performed with an oscillation frequency of 50 ± 1 Hz, a stroke length of 1 ± 0.02 mm, a load of 200 ± 1 gf and a temperature of 35 °C for 20 min.
3. Results and discussion
The variation of microhardness of base material, CCPD treated sample and MS coated samples with various number of layers is plotted inFig. 4. It shows that the highest hardness of 1371 HV0.025is achieved
by CCPD (sample A), which is significantly higher than the base ma-terial. The highest hardness of CCPD treated sample can be credited to the formation of dense structure of titanium nitride, as we already re-ported . However, the samples with MS coating (having lubricant coating of MoS2beneath the TiN layer, sample B) shows a decrease in
hardness, but still quite higher than the base material. The hardness is further decreased with the increase in number of layers of TiN and MoS2
(samples C–E). The decrease in hardness in MS coated sample can be attributed to low hardness of MoS2layer beneath hard titanium nitride
layer [17,27]. The further decrease in hardness is probably due to multiple soft MoS2layers, which have individual atomic-thin planes,
thus the sliding of planes against each other is quite easier and less resistance against deformation to the normal load .
For the improved interpretation of results, the X-ray diffraction pattern is plotted inFig. 5. The XRD pattern of base material shows the phases of steel matrix and a series of phases associated with the car-bides dispersed in the material structure. The matrix peaks correspond to Fe and the other phases are the carbides formed by alloying elements
present in the sample, as given in elemental composition inTable 1. The CCPD treated sample (sample A) with titanium cathodic cage shows the presence of Fe(N), iron nitride (ε) and titanium nitride phases. In
lit-erature, Fe(N)phase is labeled as either tetragonal nitrogen-martensite
phase  or cubic ferrite phase containing nitrogen as solid solution [29,30]. The existence of Fe(N)phase can be ascribed by the mechanism
of CCPD system, which involves simultaneous contribution of thermo-chemical diffusion of nitrogen and deposition of cathodic cage material . Due to diffusion of nitrogen in the steel matrix as solid solution, Fe(N)phase is formed. On the other hand, the presence of titanium
ni-tride phase is obviously due to sputtered-deposition of titanium from cathodic cage on the sample surface in the form of titanium nitride after reacting with nitrogen in plasma environment. The highest hardness of this sample can be ascribed to the presence of hard titanium nitride, iron nitride and solid solution of nitrogen. The MS coated sample with monolayer of each coating (sample B) depicts quite analogous phases, Fig. 3. Schematic illustration of cathodic cage plasma deposition and
multi-layer coatings by magnetron sputtering on steel substrate.
Fig. 4. Surface microhardness of base material and various treated samples.
except with the small peak of MoS2, which is due to addition of
lu-bricant coating of MoS2. The MS coated samples having multilayers of
each coating (samples C–E) indicates the presence of titanium nitride
and MoS2phases, whereas due to grazing incidence angle as well as due
to increase in thickness of coatings (combine thickness of all layers), the substrate peaks are suppressed. As the peaks of MoS2are not clearly
observable in GIXRD, thus to elucidate the peaks, Rietveld refined XRD pattern of sample B is presented inFig. 6. It is possible to verify a good relation among experimental and calculated values by the refinement quality defined by the parameter wR = 7.6%. It is reported that good refinements provide wR values in the range of 2 to 10%, thus it shows a good relationship between experimental and calculated values . This refinement depicts that the coating contains phase fraction equivalent to 84% TiN and 16% MoS2. Thus the XRD results depicts that
the hardness of MS coated samples is reduced due to the presence of soft MoS2. Although, in these coated samples top layer is of hard titanium
nitride coating, but the decrease in hardness is now clarified from XRD pattern. The XRD pattern shows that even using GIXRD, the top region contains the both phases of TiN and MoS2, which is responsible of
re-duction in hardness. Physically, this process is probably caused by inter-diffusion of both coatings elements and it can be justified later by using elemental line scan analysis.
Fig. 7shows the Raman spectra of MS coated samples with mono-layer/multilayers of each coating. The analysis performed with 633 nm laser shows the bands referring to acoustic transverse vibration (TA), longitudinal acoustic vibration modes (LA) and optical transverse (TO) due to first-order Raman vibrations . Also in the spectral range of 460–550 cm−1, second-order vibration mode 2A for samples C-E is also
observed. The vibrational band observed in the range of 360–450 cm−1
is the convolution of the peaks of first-order MoS2E2g1(383 cm−1) and
A1g(408 cm−1), in addition to the contribution of TiN bands adjacent to
these positions. Here all the first order bands are resulting from vi-brational modes within the S − Mo − S layer. The high power and energy of the laser used in the analysis induced the formation of mo-lybdenum oxide (MoO3) with peaks close to 820 cm−1and 985 cm−1
. This can be avoided by analyzing samples with low power; however, it is necessary to use this configuration to extract vibrational information from the outermost TiN layer and lubricant layer below TiN.
In order to observe surface modification by cathodic cage plasma deposition, the cross-sectional SEM image along with elemental map-ping of CCPD treated sample (sample A) is depicted inFig. 8. It shows that the modified layer is quite uniform and homogeneous, with layer thickness of around 1 ~ μm. The elemental mapping shows that the top layer mainly composed of titanium, which is obviously due to sput-tering from cathodic cage and its deposition on sample surface in the form of titanium nitride, as described by CCPD working mechanism . Furthermore, the nitrogen elemental mapping indicates that ni-trogen is diffused in the sample, which is due to processing at 400 °C, as a result nitrogen diffuses to higher depth inside the sample .
The cross-sectional SEM images along with elemental mapping (ti-tanium, nitrogen, molybdenum and sulfur) of MS coated samples are plotted inFig. 9 (as explained in experimental details section, these coatings were deposited on glass substrate for clear understanding of results). The MS coated sample with monolayer of each coating (Fig. 9a, sample B) depicts that the layers are quite uniform and dense. The Fig. 6. Rietveld refined XRD pattern of treated sample B.
Fig. 7. Raman spectrum of various treated samples.
Fig. 9. Cross-sectional BSE-SEM images along with elemental mapping (titanium, nitrogen, molybdenum, sulfur) of various treated samples (a) sample B (b) sample C, (c) sample D, (d) sample E, using the glass substrate.
uniformity of coatings is highly beneficial and demanding in forming tools applications because non-uniformity in layers causes internal tensions, and thus lifetime of tools dramatically reduced. The combined thickness of coatings is 1.97 μm, which contains the thickness of lu-bricant MoS2 coating and TiN coating. Interestingly, the elemental
mappings show the inter-diffusion of elements from one layer to the other layer (which can be further clarified by EDS elemental line scan in later section), which is probably due to processing temperature. The MS coated sample with dual layer of each coating (sample C) is shown in Fig. 9b, and it shows that the combined thickness of coatings is 4.32 μm. The MS coated sample with 4 and 6 layers (samples D–E) of each coating is depicted inFig. 9(c, d) and shows that combine thickness of coating is 9.31 and 13.7 μm. For more clarification of results, the ele-mental line scan is also performed for MS coated samples and is de-picted inFig. 10. It clearly shows that the elements of lubricant coating and titanium nitride coating are inter-diffused in each other. It depicts that the molybdenum and sulfur elements are also present in the near-surface zone of all samples, which clarifies the decrease in hardness of
samples by using combination of CCPN and MS coating. Even in the processing of all samples, the top layer is of hard titanium nitride, but this diffusion of molybdenum and sulfur in the near-surface region is probably responsible for hardness reduction. According to the Goller et al. , the individual incorporation of Mo and S atoms in the TiN lattice affects the properties of TiN, which supports our assumption that the inter-diffusion of Mo and S is responsible for decrease in hardness of MS coated samples.
Fig. 10. Elemental line scan profile (titanium, nitrogen, molybdenum, sulfur) of various treated samples (a) sample B (b) sample C, (c) sample D, (d) sample E,
Fig. 11. AFM topography of various treated samples (a) sample A, (b) sample B (c) sample C, (d) sample D, (e) sample E,
The atomic force microscopy allowed the observation of superficial morphology on the nanometer scale. The AFM scans are performed at the edges of all treated samples because these regions are of greatest interest in tool applications. The CCPD treated sample (Fig. 11a, sample A) depicts that the surface is quite homogeneous and uniform and this fact can be ascribed to the uniform deposition on the whole surface due to sputtering from cathodic cage, and this fact is already reported using titanium cathodic cage .The AFM images of MS coated samples with multiple coatings (Fig. 11 b–e, samples B–E) shows that the number of particles on the surface decreases and their size increases. Also, the height of surface nitrides is observed to increase with the increase in number of coatings. Probably, this fact is due to accumu-lation of arriving particles during coating process. The surface rough-ness of treated samples is depicted inFig. 12, which clearly shows an increase in surface roughness with increase in number of coatings.
The variation of friction coefficient as a function of sliding time is plotted in Fig. 13. It shows that the friction coefficient of the base material is high enough and rapidly varying, which can be ascribed to severe wear and low hardness . The friction coefficient is slightly reduced for CCPD treated sample (sample A), which can be credited to the high hardness of this coating, which resists against the plastic de-formation. The friction coefficient is further reduced for MS coated samples, which contains lubricant coating beneath the hard titanium nitride coating, and lowest value is achieved for MS coated sample with monolayer of each coating (sample B). This decrease in friction coef-ficient is credited to the lubricating action of MoS2coating. Although,
the samples with lubrication coatings show low hardness values, the decrease in friction coefficient is credited to lubrication, not to hardness [4,16]. While increasing the number of coatings (samples C–E), the friction coefficient starts to increasing (but still remains below than the only TiN coated sample), which can be caused by the increased roughness with increase in number of coatings. Higher the surface roughness causes higher wear rates, and thus friction coefficient starts increasing for the ceramic coatings, as reported by several authors [36,37]. However, the samples with a lubrication coating show a slight dissimilarity with this behavior. For better understanding, the average
values of friction coefficients are plotted inFig. 14. It clearly shows that the friction coefficient is lowest for the MS coated sample with mono-layer of each coating (sample B). Thus, it shows that this monomono-layer coating is more suitable due to low friction coefficient.
Besides the friction coefficient, the temperature control in the tools machining process is of notable importance in the conventional turning with tools. The measurement of temperature during the machining test shows that the films produced on the tools reduced heat loss (the heat comes from tribological contact among the tool and cast iron). The variation of temperature as a function of the number of machining steps is plotted inFig. 15. Among the various treated samples, the MS coated sample with monolayer of each coating (sample B) exhibits lowest temperature, whereas CCPD treated sample (sample A) and MS coated sample with 6 multilayers (sample E) shows the highest temperature. The highest value of temperature for CCPD treated sample can be at-tributed to the presence of abrasive hard particles in the contact in-terface. On the other hand, in the MS coated samples, the lubrication coating of MoS2is responsible for decrease in temperature. However,
for the samples with multilayer coatings, the surface is rougher and have high friction coefficient, and thus temperature is increasing with increase in number of layers . Thus, the results suggest that duplex treatment with monolayer of each coating is suitable for machining process application.
The wear tracks of various treated samples are plotted inFig. 16. It clearly demonstrates that the base material is severely worn with wide wear tracks and damaged surface appearance along the whole wear track. The CCPD treated sample (sample A) shows a decrease in the width of wear tracks as compared to base material, but the surface at high resolution depicts that the wear track is fractured. The improve-ment in wear resistance is credited to improved hardness due to the diffusion of nitrogen atoms and formation of hard TiN-coating on the surface during CCPD process. However, the coating fracture during the wear test is due to coating's brittle nature, which fails during the sliding wear in metal-to-metal contact. On the other hand, the samples with MS-coated sample with monolayer of each coating (sample B) depicts the quite uniform surface appearance and comparatively contracted Fig. 13. Friction coefficient with sliding time of base material and various treated samples.
wear track. It clearly indicates the beneficial effect of using the lu-brication coating prior to the synthesis of hard coating. However, for the samples with multiple layers of each coating (samples C–E), the wear resistance is not improved, and detrimental effects on wear re-sistance is observed by using the multiple layers. This fact is credited to the removal of hard particles from the TiN-layers, which perform as grit, and thus worsen the wear resistance as clearly evident from the wear grooves and metallic debris on the wear track, which points out abrasive wear mechanism. Thus, the observation from the wear analysis reveals that the combination of lubrication coating MoS2 and hard coating TiN is beneficial if monolayer of each coating is used, while detrimental with multiple layers.
Several authors [17,18,20] reported a decrease in wear rate using the combination of TiN and MoS2coatings, as compared to individual
hard or lubrication coating. Here we used the combination of these coatings in reverse order, by covering the lubrication coating with hard coating, and found improvement in wear resistance with single layer of each coating. However, with the multilayer coatings, the wear
resistance is deteriorated due to the presence of hard particles removed from the hard coating, which causes abrasive wear. The combined ac-tion of lubricaac-tion and hard coating is efficient due to inter-diffusion of both coatings elements, and thus both hardness and wear resistance are attained. Additionally, in this study the pre-treatment by cathodic cage plasma deposition produces a hardness gradient and thus less removal of coatings from the surface during wear test, which points out better adhesion with substrate.
Fig. 14. Average friction coefficient of base material and various treated
Fig. 15. Variation of tool-work interface temperature as a function of the ma-chining step.
Fig. 16. SEM images of wear tracks of (a) base material, (b) sample A, (c) sample B, (d) sample C, (e) sample D, (f) sample E.
4. Conclusion and final remarks
In this study, the combined effect of hard titanium nitride (TiN) coating and lubrication coating of molybdenum disulfide (MoS2) by
magnetron sputtering on cathodic cage plasma nitrided M2 tool steel is investigated. The conclusions of this study are as follows:
1. The cathodic cage plasma deposition technique with titanium cathodic cage can be utilized effectively for the significant im-provement in hardness, with no requirement of high vacuum level and comparatively low processing cost. The hardness starts to de-crease by introducing the solid lubrication coating of MoS2. The
hardness appears to be decreasing with the increase in the number of coating layers.
2. The sample treated by cathodic cage plasma deposition depicts the appearance of multiple phases of titanium nitride and iron nitrides along with nitrogen diffused martensite phase. The magnetron sputtered coated samples shows the presence of titanium nitride and molybdenum disulfide phases.
3. The Raman spectrum also supports the formation of titanium nitride bands and convolution peaks of the first-order MoS2.
4. The cross-sectional SEM analysis depicts the formation of homo-geneous and uniform multilayers of TiN and MoS2. This uniformity
of coatings is highly valuable in forming tools applications because non-uniformity in layers causes internal tensions, and thus the lifetime of tools dramatically reduced. The elemental mapping and elemental line scan shows the inter-diffusion of elements in the layers, which is probably responsible for lubricating performance of MoS2, even it is underneath the TiN hard coating.
5. The friction coefficient is significantly reduced by using solid lu-brication coating, particularly using monolayer of each coating. As a result, the temperature measured in the machining test is also re-duced for this sample.
The results obtained in this study clearly reveal that the cathodic cage plasma deposition can be used effectively for hardness improve-ment and magnetron sputtering for the combination of solid lubrica-tion/hard coating. Therefore, using this combination of treatment, the friction coefficients and thus failure to coatings can be minimized. These coatings can easily be synthesized on machining tools, and thus results are predicted to be valuable in all machining applications on industrial scale.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influ-ence the work reported in this paper.
The authors want to express their gratitude to Departamento de Materiais—DEMAT, Laboratório de Materiais Multifuncionais, Experimentação Numérica da Escola de Ciências e Tecnologia sECT da Universidade Federal do Rio Grande do Norte—UFRN, and Grupo de Crescimento de Cristais e Materiais Cerâmicos da USP‐São Carlos. References
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