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Effect of hBN and SiC addition on laser assisted processing of ceramic matrix composite coatings
Journal Pre-proof Effect of hBN and SiC addition on laser assisted processing of ceramic matrix composite coatings Debjit Misra, Vaibhav Nemane, Suman Mukhopadhyay, Satyajit Chatterjee PII:
S0272-8842(19)33747-2
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.245
Reference:
CERI 23898
To appear in:
Ceramics International
Received Date: 9 October 2019 Revised Date:
23 December 2019
Accepted Date: 27 December 2019
Please cite this article as: D. Misra, V. Nemane, S. Mukhopadhyay, S. Chatterjee, Effect of hBN and SiC addition on laser assisted processing of ceramic matrix composite coatings, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.245. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Effect of hBN and SiC Addition on Laser Assisted Processing of Ceramic Matrix Composite Coatings Debjit Misraa, Vaibhav Nemanea, Suman Mukhopadhyayb, Satyajit Chatterjeea,1 a
Discipline of Mechanical Engineering, Indian Institute of Technology Indore, Simrol, Indore 453552, India b Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Indore 453552, India Abstract To develop TiB2-TiN-SiC and TiB2-TiN-SiC-hBN based ceramic matrix composite coatings with superior mechanical and tribological properties suitable for engineering applications, a high-power CW-laser is used to initiate chemical reactions in precursor powder mixtures (TiO2, SiO2, hBN and graphite) of several compositions preplaced over AISI1025 steel substrates. Here, coatings’ stoichiometric compositions (TiB2-TiN-SiC) fabricated through in-situ synthesis are altered by adding SiC and additional amount hBN in the precursor at variable weight ratios. Presence of variable amount of SiC in TiB2-TiN-SiC and hBN in TiB2-TiN-SiC-hBN composites imparts several microstructural, mechanical and tribological property combinations. In this study, an attempt has been made to experimentally evaluate the optimum range of hBN and SiC additions to obtain highest improvements in lubrication and wear resistance respectively.
Keyword: Ceramic matrix composite; Laser surface alloying; Specific wear rate; Friction coefficient; Hardness
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Corresponding author. E-mail address: [email protected] (SatyajitChatterjee).
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Introduction Engineering applications in modern era demand greater attention to friction coefficient and wear rate, the tribological properties of a material. These are not the intrinsic properties of materials but are regarded as characteristics of the engineering system (tribo-system) in concern. Friction and wear are to be treated as two responses obtained from one tribo-system and can be efficiently controlled by the application of multi-phase composite coatings in addition to traditional lubrication methods. Coatings consisting of solid lubricants, in order to produce self-lubricating systems, do not require any externally applied lubrication. Over the years, the technologies of producing self-lubricating systems along with solid lubricants have advanced rapidly in various sectors of engineering and can now be accomplished in several forms of self-lubricating composites in coatings and films [1-3] Hexa-boron nitride, with its lamellar crystalline structure and van der Waals forces acting as interplanar bonding, is a good exponent for solid lubrication. Solid lubricants have the characteristics of being weak in shear and in solid crystalline lubricants, shear occurs by slip propagating along preferred crystallographic planes, providing lubrication. It is observed in the published reports that presence of hBN in composites is helpful in reducing friction coefficient but can also hinder achieving such tribological properties [4-8]. It is a tough challenge to achieve all mechanical and tribological advantages in a composite by the addition of hBN. Several compositions are attempted by various researchers proposing addition of specific amounts of hBN in composites for the simultaneous improvement of mechanical and tribological properties. Tharajak et al. developed a composite with 8wt.% hBN (particle size 1µm) and achieved significant improvement in both friction and wear [9]. Chi et al. added 15wt% hBN in TiB2/2024Al composites for the improvement of the tribological properties [10]. Zhang et al. noticed that self-lubricating properties up to 800oC can be achieved by using 16wt % hBN in the composite coating [11]. Hsiao et al. used 10wt%hBNfor the enhancement of lubrication
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[12]. Kitiwan et al. noticed that at optimum composition (15vol%hBN), composites exhibited high hardness and fracture toughness [13]. Lu et al. observed laser clad Ni60-10%hBN coatings to exhibit better tribological and mechanical behavior [14]. Silicon carbide (SiC) can play a vital role in improving the mechanical and tribological properties of composites. SiC in composites can improve hardness [15-16] wear resistance [17] and toughness along with resistance to oxidation and thermal shock [18]. Literature also suggests that the addition of larger amount of hard particle causes degradation of the overall mechanical properties. Jianxin reported the fracture toughness of a composite to steadily increase with the rise in SiC whisker content up to 30 vol.% and decrease beyond it [19]. Denget al. observed the strength of composites to decrease with the presence of higher amount SiC [20]. Several researchers attempted to strike a perfect balance between the reinforcing phases and matrix for that composite to be suitable for engineering applications [12-13, 20]. It is understood from the previously published research that composites consisting of multiple reinforcements achieve marked improvement in mechanical properties as compared to singlephase ceramics. Ternary phase ceramic composites are better alternatives as compared to monolithic or binary phase ceramic composites owing to their superior mechanical properties [21-23]. Composites developed with TiB2, TiN, and SiC can display different characteristics, based on the composition of the constituents. A predominance of SiC helps in achieving suitability for structural and corrosion related applications. Engineering applications demand improvement in both mechanical and tribological properties and developing a composite coating with both good lubricating properties and superior mechanical properties is a challenge. In our previous communications, studies on insitu composite coatings, developed with TiB2, TiN and SiC through laser surface alloying are reported [23, 24]. High power CW-laser initiated a high temperature chemical reaction in the
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precursor powder mixture of the reactant phases of Eq. (1) preplaced over steel substrates to develop TiB2-TiN-SiC composite coating with stoichiometric composition [23]. On successful incorporation of hBN in free form in the coating matrix as a self-lubricating phase helped in imparting significant changes in coating’s frictional properties. But due to addition of such a softer phase, composite’s response towards dynamic loading weakened and a much lower resistance to wear at sliding, owing to its lower hardness, is observed. At the same time, the composite developed with more than stoichiometric amount of SiC is observed to have improved in terms of their mechanical properties but significant rise in the values of friction coefficient is also noted. The present study focuses on the development of two different categories ofin-situ TiB2-TiN-SiC composite coatings with the incorporation of free hBN and more than stoichiometric amount of SiC without negatively affecting any of its mechanical and tribological properties. To do this, several TiB2-TiN-SiC composites with variable amounts of hBN are fabricated and thoroughly characterized to find out a combination perfectly suitable for engineering applications. Few more TiB2-TiN-SiC samples processed with extra amount of SiC are also tested to achieve a suitable composition with much improved mechanical properties without compromising on tribological properties.
3TiO2 + 2hBN + SiO2 +C = TiB2 + 2TiN + SiC + 4O2
(1)
Experimental procedure Eq. (1) is followed to make a precursor powder mixture containing TiO2, SiO2, hBN and graphite to preplace over surface ground and cleaned AISI1025 steel substrates [size: 100×50×8 mm3] maintaining uniformity in thickness for the preparation of coatings. A highpower CW-fiber laser beam (2 kW laser power) is used in a scanning mode to raise the surface temperature of the samples that initiates a chemical reaction in the preplaced precursor powder mixture. The reaction products of Eq. (1) (titanium-di-boride, titanium
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nitride, silicon carbide) on subsequent laser treatment, develop a ceramic matrix composite layer with in-situ reinforcements over steel substrates. The composite coating processed with stoichiometric composition according to Eq. (1) is identified as K1. Following the same route, 10 more coatings of different compositions are developed (compositional details are mentioned in Table 1). For samples K2-K8, amount of hBN is increased in the precursor powder mixtures in seven weight increments with respect to sample K1. Similarly, K9-K11 samples are produced with three weight increments of SiC added to the reactant phases in the precursor mixture (TiO2, SiO2, hBN, and graphite powder) with a view to have more SiC than stoichiometric amount in the final coating. Here, the amount of SiC to be added is calculated with respect to the stoichiometric amount of SiC (according to Eq.(1)) that is supposed to be produced in-situ in sample K1. Thorough microstructural studies on developed composite coatings are performed using the scanning electron microscope (SEM) (Zeiss Supra-55and Zeiss EVO 18 Research). Wire-EDM (Electronica-ECOCUT) is used to cut smaller specimens (10×10×8 mm3) from the coated samples for the microstructural studies. X-ray diffractometer (Bruker D2 PHASER) is used under Cu-K radiation to carry out phase analyses of the composite coatings. Also, phases are α
determined with bright field imaging and selected area electron diffraction (SAED) patterns observed in a JEOL JEM 2100 HRTEM (high-resolution transmission electron microscope). Lab-RAMAN Evolution 300 Raman Spectrometer is used for Raman scattering. Tribological properties of composites are evaluated by a linear reciprocating tribometer (DUCOM CM 9104). For the tribological tests, a Wire cut specimen (20×20×8 mm3) from the bulk coated sample is used as a square block against the counterbody. Details of the parameters of the tribological test are enlisted in Table 2. Hardness test has been performed on the cross section of the composite coating using hardness tester ((INNOVA, Falcon 500, Maastricht, The Netherlands). 5
Results and Discussion Microstructural details of the ceramic matrix composite coatings are observed under Scanning electron microscope (Figure 1(a-b)). The magnified views of the representative coating’s cross-section (K2), as shown in Figure 1(a), reveal the distribution of reinforcements in the coating matrix at the cross-section.
SEM images taken in back
scattered emission (BSE) mode indicate towards formation of several phases in the coating. TiB2, TiN and SiC phases (as mentioned in Eq. 1) backscatter differently under the SEM owing to the difference in their average atomic numbers. This results variations in image brightness, which elucidates the distribution of phases in the coating microstructure. EDS analyses showing the presence of Ti, B, N, Si, C and Fe denote dispersion of titanium-rich phases in silicon-rich matrix. The higher magnification of the labeled area of sample K2 shown in the inset of Figure 1(a) portrays a compact and homogeneously distributed microstructure with finer reinforcements. Factors relating to temperature gradient, solidification front velocity and cooling rate play vital roles in determining these microstructural features. Figure 1(b) shows in BSE, the dispersion of white acicular reinforcements in a gray continuous matrix at the cross section of sample K10. Figure 2(a-b) shows bright field image and selected area electron diffraction (SAED) patterns of the composite coating K4 obtained under transmission electron microscope (TEM). The SAED pattern analysis reveals presence of nanocrystalline hBN and TiN in the sample. Diffraction patterns of composite coatings obtained from XRD analyses (Figure 3(a-b)) show the presence of SiC, TiN and TiB2 in all the composite coatings (K1-K11). Traces of hBN are also obtained in samples K2-K8 (Fig 3(a)). Raman spectrum of the sample K1 is obtained over wide range of frequency (up to 450 cm-1) and is presented in Figure 4. The strongest peaks observed at 210, 292 and 343 cm-1 can be associated with the presence of titanium nitride (TiN) which is almost same as those 6
previously reported [25-29]. The low-frequency scattering originates due to transverse and longitudinal acoustical phonons [25, 28]. The presence of TiB2 is confirmed by observing peaks at 271 and 401 cm-1 in Figure 4. These Raman shifts indicating the formation of TiB2 are in good agreement with a previously reported analysis of Raman spectroscopy [28, 30,31]. The present work focuses on the investigation of mechanical and tribological properties of composite coatings developed with varying amounts of hBN as solid lubricant and SiC as wear resistive material in the precursor mixture. The evaluation of the tribological properties of composite coatings K1–K11 has been carried out in a linear reciprocating tribometer at a constant load, under a dry sliding wear condition against WC-6%Co counterbody. The details of the tribological tests performed are mentioned in Table 2. Friction coefficient obtained on addition of different amounts of hBN and SiC in the precursor mixture are presented in Figure 5(a-b). After the initial run-in period, as the quasi-steady state is attained, the values of friction coefficient are noted at all the tests. XRD analysis already confirmed the presence of hBN in composite coatings K2-K8. It is observed from Figure 5(a) that increase in the amount of hBN in precursor causes reduction in friction coefficient only upto K6 and further increment in hBN content results rise in friction coefficient values. Increasing the amount of SiC in coatings beyond stoichiometric composition (K9-K11) also causes rise in friction coefficient (Figure 5(b)). Values of specific wear rate in sliding wear tests conducted are depicted in Figure 6(a-b). Effect of variations in the amount of hBN in precursor mixtures used to fabricate the composite coatings on specific wear rate shows an interesting trend (Figure 6(a)). A small rise observed in specific wear rate values in composite coatings K1–K5 can be corroborated to the increasing amount of hBN in precursor and beyond certain percentage weight addition of hBN, values of specific wear rate (K6-K8) increase even faster. Increase in the amount of
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SiC in the composite coatings (K9-K11) causes a gradual reduction in specific wear rate values (Figure 6(b)). Figure 7 represents cross-sectional microhardness of all composite coatings developed in the present study. It reveals that with the rise in hBN content up to certain level (between K2 and K5), minor reduction in hardness values are observed. Beyond this, composite coatings K6K8 with even higher amount of hBN, show reduction in hardness values at significantly faster rate. Similarly, additional amount of SiC in composite coatings K9 and K10, causes an increment in hardness values. With even higher amount of SiC, K11 display decrease in the hardness value. Friction coefficient in a situation of sliding wear is influenced by many factors and is evaluated by summing up the contributions of adhesion, elasto-plastic deformation and the presence of tribo-films respectively [32-33]. For composites, tribological behavior is affected significantly by the participation of wear debris in the interaction with a counterbody and environmental effects such as temperature and humidity [34]. However, in the present study the tribological tests are conducted in constant temperature and humidity and hence the effects are uniform to all the samples. From the XRD analyses, presence of highly-ordered crystalline hBN in coated samples K2K8 is confirmed. This corroborates to detection of an intense peak at 26.6° (2θ) of the diffraction data, assigned to the crystallographic plane (002) of hBN [35]. Similar peak is observed at 2θ ∼ 26° of the hexagonal graphitic structure [36]. A lamellar structure similar to that of graphite is present in hBN with boron and nitrogen atoms having covalent bonds in between, forming hexagonal B3N3 cells and two-dimensional BN layers stacked with weak van der Waals forces [37]. The spacing (0.34 nm) in between these layers is very much similar to that in graphite (0.335 nm). This lamellar structure of hBN is weak in shear and display lubrication properties in a tribological interaction. In this study, presence of the hBN 8
in the composite coatings K2-K8 caused reduction in friction coefficient as compared to the composite coatings K1 and K9-K11. Interestingly, increasing amount of hBN shows a decreasing trend of friction coefficient within K2-K6 and on further increase in hBN content in K7-K8; the trend seems to have reversed with the rise in friction coefficient values. Here, the presence of certain higher amount of hBN resulted in the weakening of bond strength between the coating matrix and reinforcements which may overweigh the solid lubricant effect and nucleate more cracks along with deep grooves in the composite coating resulting easier dislodgement of particles [38]. At the zone of tribological interaction, dislodged particles of coatings trapped between test sample and counterbody result in a significant rise in friction coefficient. Worn surface morphologies observed under SEM display higher amount wear debris along with some fracture pits on sample K7 and K8. A composite, containing higher percentage of softer phases like hBN in its matrix, on being subjected to a higher contact pressure at the zone of tribological contact in a sliding wear test produce gross surface and subsurface deformation and cracking. As a result, generation of debris and subsequent formation of fracture pits of considerably larger dimensions are observed. The sample surfaces (K7-K8) get repeatedly grounded by wear debris forming fresh surfaces of interaction and the worn surfaces show severe damage. In such conditions, wear mechanism is observed to be dominated by abrasive wear and higher values of friction coefficient are recorded [39]. Crystallographic orientation and other bulk properties are found to influence material’s frictional response. Narrower dislocations are formed due to the presence of a significantly high amount of hard SiC particles with covalent bond. This leads to the increase in friction coefficient of composite coatings K9-K11. Values of specific wear rate of composite coatings K2-K5 show minor increase because of the presence of hBN. Soft and lubricious hBN in the coatings may reduce brittle fracture remarkably and result a smoother worn surface after the sliding wear test. The mode of
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fracture in composites change from trans-granular to inter-granular due to the presence of hBN causing the fracture toughness to rise. The deflection of the crack at the hBN interface can explain such improvement [38]. A tougher matrix of composite reduces crack nucleation, which eventually helps in improving its wear resistance properties. Increment in hBN amount further to the sample K5, as in samples K6 and K7, shows a significant rise in specific wear rate. Presence of certain higher amount of hBN in composites leads to weakening of matrix and results greater loss of material under the dynamic loading condition of tribological interactions. Here, the softer hBN, upon agglomeration, tends to form zones of continuous hBN phase, augmenting nucleation and propagation of tensile cracks during tribological interactions [13]. This causes the composite to lose the tribological advantage of having hBN in its matrix [38]. Microhardness is a key factor in establishing composites’ tribological properties. Coated samples K2-K5 are observed to have similar hardness values and on further increase in hBN amount (in K6-K8) significant degradation in hardness is observed. On the other hand, modification in grain boundary strength and reduction in the surface damage associated with intergranular fracture are observed due to the presence of higher amount of SiC in composites [40]. Movement of narrower dislocations formed in the coating matrix in presence of SiC occurs only under larger values of stresses [23]. Asl et al. noticed that improvement in density and microstructural characteristics are obtained with higher amount of SiC in composites [41]. All these result in wear resistance to improve significantly in composites developed with the excess amount of SiC in precursor. Figure 8 shows wear track morphologies of composite coatings K1-K11 generated in sliding wear tests. The worn-out surfaces on samples K1-K5 suggest domination of adhesive wear with occurrences of delamination happening in presence of both single and repeated cycle deformation mechanisms under the cyclic stresses of tribological tests. Visible signs of
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abrasive wear on K1-K5 are lesser as compared to that in samples K6-K8. Smaller quantities of debris formed during tribological interactions augment abrasive wear. At higher hBN content, degradation of mechanical properties of ceramic composites leads to spallation and pitting. The wear debris retaining at the wear track get accumulated in these surface defects and weaken the effect of abrasive wear. Wear tracks on composite coatings K6-K8 show formation of deeper grooves, larger pits and wider cracks, which correspond to higher specific wear rate and corroborates the disadvantages imparted by the presence of higher amount of hBN in the coating matrices. Dislodgement at the coated surface and resultant material losses are much less abrupt in the composite coatings K9-K11, fabricated with higher amount of SiC. Surface deformation and damage at smaller scales make the surfaces of tribological interactions appear relatively smoother as compared to other coatings. Conclusion: A precursor powder mixture, prepared following the stoichiometry of a high-temperature chemical reaction, is preplaced over a steel substrate and subsequently laser treated to develop an in-situ TiB2-TiN-SiC composite coating. Seven more compositions of precursors with more than stoichiometric amount of hBN are used to produce TiB2-TiN-SiC-hBN coatings with varying weight percentages of hBN. Similar method is used to fabricate TiB2TiN-SiC coatings with variable amounts of SiC. SEM studies portray the composite coatings’ compact and homogeneously distributed microstructures with finer reinforcements developed without any form of discontinuities. XRD analyses show the presence of all expected phases in composite coatings developed with the combination of high-temperature chemical reaction and laser treatment. The stoichiometric composition of in-situ TiB2-TiN-SiC composite coating appears to be good in terms of hardness and tribological properties. Enhancement in friction and wear behaviors can be correlated to the presence of free-hBN in composite matrices. Remarkable changes in mechanical and tribological properties are noticed with the 11
increase in hBN content. Interestingly, advantages are obtained only up to a certain weight percentage of addition, and beyond that degradation in friction and wear resistance is observed. In the present work, improvement in frictional properties is achieved on addition of hBN up to K6 composite coating. The lamellar structure of hBN (sp2 hybridization) is weak in shear and displays lubrication properties in tribological interactions. In K7-K8, the trend seems to have reversed. Here significant rise in the values of friction coefficient is observed because of the presence of certain higher amount of hBN. This resulted in the weakening of bond strength between the coating matrix and reinforcements which overweighs the effect of solid lubrication, resulting in easier dislodgement of particles. Dislodged particles of coatings trapped between test sample and counterbody also contribute to rise in friction coefficient. Addition of free-hBN in composite coating K2-K5 shows minor change in specific wear rate with small change in hardness. This tribological advantage of solid lubrication is lost in samples K7 and K8 because of the presence of certain higher amount of hBN in composites leading towards weakening of matrix. This results in greater loss of material under the dynamic loading condition of tribological interactions. Similarly, increasing the SiC content in TiB2-TiN-SiC matrix beyond the stoichiometric composition affects the hardness and frictional properties. In this study, coatings K9-K10 show significant improvement in hardness and wear resistance and further addition of SiC shows reversal of the trends in the hardness of the composite. This study presents a wholesome idea of the property tailoring of TiB2-TiN-SiC composites affected by hBN and SiC. Acknowledgments The authors are thankful to Sophisticated Instrument Centre (SIC) and Discipline of Physics of IIT Indore. Facilities received from Department of Science and Technology, Government. of India, under FIST scheme (grant number SR/FST/PSI-225/2016) is highly acknowledged. Authors are grateful to Central Workshop and AMP Laboratory of IIT Indore, for material 12
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[30] U. D. Wdowik, A. Twardowsk, B. Baw Rajchel, Vibrational Spectroscopy of Binary Titanium Borides: First-Principles and Experimental Studies, Adv Cond Matter Phys. 18(2017) 1-9. http://dx.doi.org/10.1155/2017/4207301 [31] S. Sahoo, S.K. Singh, Synthesis of TiB2 by extended arc thermal plasma, Ceram. Int. 43(2017) 15561-15566. http://dx.doi.org/10.1016/j.ceramint.2017.08.108 [32] B. Maria Maros, A. K.NemethZoltan Karoly, EszterBodis, ZsoltMarosOrsolyaTapaszto, KatalinBalazsi,
Tribological
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nanocomposites produced by HIP and SPS technology, Tribol. Int. 93(2016) 269-281. https://doi.org/10.1016/j.triboint.2015.08.041 [33]J. Zhang, S. Yang, Z. Chen, M. Wyszomirska, J. Zhao, Z. Jiang, Microstructure and tribological behaviour of alumina composites reinforced with SiC-graphene core-shell nanoparticles, Tribol. Int. 131(2019) 94-101. https://doi.org/10.1016/j.triboint.2018.10.009 [34] J. Larsen-Basse, Basic theory of solid friction, ASM Handbook, vol. 18 (1992). Ed: Blau. [35] Xiao-kun Ma, Nam-Hee Lee, Hyo-Jin Oh, Sang-Chul Jung, Won-Jae Lee, Sun-Jae Kim, Morphology control of hexagonal boron nitride by a silane coupling agent, J Cryst. Growth 316 (2011), 185-190. https://doi.org/10.1016/j.jcrysgro.2010.12.066
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[36] OdedHod, Graphite and Hexagonal Boron-Nitride have the Same Interlayer Distance. Why?, J Chem Theory Comput, 8 (2012), 1360-1369. https://doi.org/10.1021/ct200880m [37] N. KostoglouK. PolychronopoulouC. Rebholz, Thermal and chemical stability of hexagonal boron nitride (h-BN) nanoplatelets, Vacuum, 112(2015), 42-45. https://doi.org/10.1016/j.vacuum.2014.11.009 [38] L. Du, W. Zhang, W. Liu, J. Zhang, Preparation and characterization of plasma sprayed Ni3Al–hBN
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19
List of Table Caption Table 1: Details of ceramic matrix composite coatings developed. Table 2: Operating conditions for tribological characterization
List of Figure Caption Figure 1: SEM images of cross-sections of representative composite coatings (a) K2 and (b) K10 (magnified view of the labeled area on the cross-sectional view of K2 is given in the inset). Figure 2: TEM image of K4 sample showing the (a) top surface morphology and corresponding (b) Selected area electron diffraction (SAED). Figure 3: X-Ray diffraction spectra of composite coatings. (a) K1-K8 and (b) K9-K11. Figure 4: Raman spectrum of sample K1 showing the presence of TiN and TiB2 phases. Figure 5: Variations in friction coefficients of laser surface alloyed composite coatings developed with compositional variations (a) K1-K8 and (b) K1, K9-K11 Figure 6: Variations in specific wear rate of different composite coatings developed with compositional variations (a) K1-K8 and (b) K1, K9-K11. Figure 7: Vickers Hardness of different composite coatings developed with compositional variations. Figure 8: SEM images showing morphologies of wear tracks formed on composite coatings K1-K11 at 12.5 N normal load.
20
Table 1: Details of ceramic matrix composite coatings developed Samples Nomenclature
K1 K2 K3 K4
CMC coatings deposited on steel substrate
Composition of pre-placed powder layer TiO2 SiO2 C hBN SiC (g) (g) (g) (g) (g) TiB2+TiN+SiC 10 2.5 0.5 2.06 --Additional wt.% hBN in precursor 2.36 --2.68 ----3.02
K5
3.54
---
4.1
---
K7
4.8
---
K8
5.36
---
K6
TiB2+TiN+SiC+hBN
10
2.5
0.5
Additional wt.% SiC in precursor 0.76
K9 K10 K11
TiB2+TiN+SiC
10
21
2.5
0.5
2.06
1.84 2.96
Table 2: Operating conditions for tribological characterization Load
Stroke
Frequency
WC-Co Ball diameter
Test time
(N)
(mm)
(Hz)
(mm)
(Min)
12.5
3
20
6
15
22
Figure 1: SEM images of cross-sections of representative composite coatings (a) K2 and (b) K10 (magnified view of the labeled area on the cross-sectional view of K2 is given in the inset).
23
TiN
hBN
TiN hBN
(a)
20nm
(b)
Figure 2: TEM image of K4 sample showing the (a) top surface morphology and corresponding (b) Selected area electron diffraction (SAED).
.
24
-TiN
- SiC
- TiB2
Cu-Kα
- hBN
(a)
-TiN
K8
- SiC
- TiB2
Cu-Kα
(b)
Intensity (a. u.)
Intensity (a. u.)
K7 K6 K5 K4 K3
K11
K10
K2 K1 20
30
40
50
60
70
K9 20
80
30
40
50
60
70
80
2θ (degree)
2θ (degree)
Figure 3: X-Ray diffraction spectra of composite coatings. (a) K1-K8 and (b) K9-K11.
25
Intensity/Counts
Fit Peak 1 Fit Peak 2 Fit Peak 3 Fit Peak 4 Fit Peak 5 Cumulative Fit Peak Data
150
200
250
300
350
400
450
Raman shift (cm-1)
Figure 4: Raman spectrum of sample K1 showing the presence of TiN and TiB2 phases.
26
1.0 0.9
1.0
(a)
Stroke length: 3 mm
0.7 0.6 0.5 0.4 0.3 0.2 K1
K2
K3
K4
K5
K6
K7
K8
0.0
Test load: 12.5 N
(b)
Stroke length: 3 mm
0.8
Friction coefficient
Friction coefficient
0.8
0.1
0.9
Test load: 12.5 N
0.7 0.6 0.5 0.4 0.3 0.2 0.1
K1
K9
K10
K11
0.0
Composite coating
Composite coating
Figure 5: Variations in friction coefficients of laser surface alloyed composite coatings developed with compositional variations (a) K1-K8 and (b) K1, K9-K11
27
6
Specific wear rate (10-5mm3/N-m)
Specific wear rate (10-5mm3/N-m)
7
Test load: 12.5 N
(a)
Stroke length: 3 mm Frquency: 20 HZ Ball: WC-Co (6mm)
5 4 3 2 1 K1
K2
K3
K4
K5
K6
K7
K8
0
Composite coatings
4.0 3.5
Test load: 12.5 N
(b)
Stroke length: 3 mm Frquency: 20 HZ
3.0
Ball: WC-Co (6mm)
2.5 2.0 1.5 1.0 0.5 K1
0.0
K9
K10
K11
Composite coatings
Figure 6: Variations in specific wear rate of different composite coatings developed with compositional variations (a) K1-K8 and (b) K1, K9-K11
28
Vickers Hardness (HV0.10)
2400 2200 2000 1800 1600 1400 1200 1000
K1
K2
K3
K4
K5
K6
K7
K8
K9
K10
K11
Composite Coatings
Figure 7: Vickers Hardness of different composite coatings developed with compositional variations.
29
K2
K1
Debris
Scratch Pit
Delamination
20 µm
20 µm
K3
Debris Pit
Delamination
20 µm
Scratch
K4
K5
Debris
Pit
K7 Spallation
Pit
20 µm K9
K8 Pit
Pit Debris
Spallation Debris
20 µm Groove
Spallation
20 µm
20 µm
Debris
K6
Scratch
Spallation
20 µm
Micro cracks
K10
20 µm
K11 Pit
Delamination Scratch
Scratch
Debris
20 µm
20 µm
Figure 8: SEM images showing morphologies of wear tracks formed on composite coatings K1-K11 at 12.5 N normal load
30
To The Editor Ceramic International Dear Sir, Please find the attached manuscript titled “Effect of hBN and SiC Addition on Laser Assisted Processing of Ceramic Matrix Composite Coatings” for the consideration for publication in the ‘Ceramic International’. On the behalf of the authors, I hereby confirm that there are no known conflicts of interest associated with this publication.We also confirm that our work is original and has neither been published elsewhere nor is currently under consideration for publication in any other place. Thank you Sincerely,
Satyajit Chatterjee Corresponding Author Associate Professor Discipline of Mechanical Engineering Indian Institute of Technology Indore, India Email: [email protected]
S0272-8842(19)33747-2
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.245
Reference:
CERI 23898
To appear in:
Ceramics International
Received Date: 9 October 2019 Revised Date:
23 December 2019
Accepted Date: 27 December 2019
Please cite this article as: D. Misra, V. Nemane, S. Mukhopadhyay, S. Chatterjee, Effect of hBN and SiC addition on laser assisted processing of ceramic matrix composite coatings, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.245. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Effect of hBN and SiC Addition on Laser Assisted Processing of Ceramic Matrix Composite Coatings Debjit Misraa, Vaibhav Nemanea, Suman Mukhopadhyayb, Satyajit Chatterjeea,1 a
Discipline of Mechanical Engineering, Indian Institute of Technology Indore, Simrol, Indore 453552, India b Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Indore 453552, India Abstract To develop TiB2-TiN-SiC and TiB2-TiN-SiC-hBN based ceramic matrix composite coatings with superior mechanical and tribological properties suitable for engineering applications, a high-power CW-laser is used to initiate chemical reactions in precursor powder mixtures (TiO2, SiO2, hBN and graphite) of several compositions preplaced over AISI1025 steel substrates. Here, coatings’ stoichiometric compositions (TiB2-TiN-SiC) fabricated through in-situ synthesis are altered by adding SiC and additional amount hBN in the precursor at variable weight ratios. Presence of variable amount of SiC in TiB2-TiN-SiC and hBN in TiB2-TiN-SiC-hBN composites imparts several microstructural, mechanical and tribological property combinations. In this study, an attempt has been made to experimentally evaluate the optimum range of hBN and SiC additions to obtain highest improvements in lubrication and wear resistance respectively.
Keyword: Ceramic matrix composite; Laser surface alloying; Specific wear rate; Friction coefficient; Hardness
1
Corresponding author. E-mail address: [email protected] (SatyajitChatterjee).
1
Introduction Engineering applications in modern era demand greater attention to friction coefficient and wear rate, the tribological properties of a material. These are not the intrinsic properties of materials but are regarded as characteristics of the engineering system (tribo-system) in concern. Friction and wear are to be treated as two responses obtained from one tribo-system and can be efficiently controlled by the application of multi-phase composite coatings in addition to traditional lubrication methods. Coatings consisting of solid lubricants, in order to produce self-lubricating systems, do not require any externally applied lubrication. Over the years, the technologies of producing self-lubricating systems along with solid lubricants have advanced rapidly in various sectors of engineering and can now be accomplished in several forms of self-lubricating composites in coatings and films [1-3] Hexa-boron nitride, with its lamellar crystalline structure and van der Waals forces acting as interplanar bonding, is a good exponent for solid lubrication. Solid lubricants have the characteristics of being weak in shear and in solid crystalline lubricants, shear occurs by slip propagating along preferred crystallographic planes, providing lubrication. It is observed in the published reports that presence of hBN in composites is helpful in reducing friction coefficient but can also hinder achieving such tribological properties [4-8]. It is a tough challenge to achieve all mechanical and tribological advantages in a composite by the addition of hBN. Several compositions are attempted by various researchers proposing addition of specific amounts of hBN in composites for the simultaneous improvement of mechanical and tribological properties. Tharajak et al. developed a composite with 8wt.% hBN (particle size 1µm) and achieved significant improvement in both friction and wear [9]. Chi et al. added 15wt% hBN in TiB2/2024Al composites for the improvement of the tribological properties [10]. Zhang et al. noticed that self-lubricating properties up to 800oC can be achieved by using 16wt % hBN in the composite coating [11]. Hsiao et al. used 10wt%hBNfor the enhancement of lubrication
2
[12]. Kitiwan et al. noticed that at optimum composition (15vol%hBN), composites exhibited high hardness and fracture toughness [13]. Lu et al. observed laser clad Ni60-10%hBN coatings to exhibit better tribological and mechanical behavior [14]. Silicon carbide (SiC) can play a vital role in improving the mechanical and tribological properties of composites. SiC in composites can improve hardness [15-16] wear resistance [17] and toughness along with resistance to oxidation and thermal shock [18]. Literature also suggests that the addition of larger amount of hard particle causes degradation of the overall mechanical properties. Jianxin reported the fracture toughness of a composite to steadily increase with the rise in SiC whisker content up to 30 vol.% and decrease beyond it [19]. Denget al. observed the strength of composites to decrease with the presence of higher amount SiC [20]. Several researchers attempted to strike a perfect balance between the reinforcing phases and matrix for that composite to be suitable for engineering applications [12-13, 20]. It is understood from the previously published research that composites consisting of multiple reinforcements achieve marked improvement in mechanical properties as compared to singlephase ceramics. Ternary phase ceramic composites are better alternatives as compared to monolithic or binary phase ceramic composites owing to their superior mechanical properties [21-23]. Composites developed with TiB2, TiN, and SiC can display different characteristics, based on the composition of the constituents. A predominance of SiC helps in achieving suitability for structural and corrosion related applications. Engineering applications demand improvement in both mechanical and tribological properties and developing a composite coating with both good lubricating properties and superior mechanical properties is a challenge. In our previous communications, studies on insitu composite coatings, developed with TiB2, TiN and SiC through laser surface alloying are reported [23, 24]. High power CW-laser initiated a high temperature chemical reaction in the
3
precursor powder mixture of the reactant phases of Eq. (1) preplaced over steel substrates to develop TiB2-TiN-SiC composite coating with stoichiometric composition [23]. On successful incorporation of hBN in free form in the coating matrix as a self-lubricating phase helped in imparting significant changes in coating’s frictional properties. But due to addition of such a softer phase, composite’s response towards dynamic loading weakened and a much lower resistance to wear at sliding, owing to its lower hardness, is observed. At the same time, the composite developed with more than stoichiometric amount of SiC is observed to have improved in terms of their mechanical properties but significant rise in the values of friction coefficient is also noted. The present study focuses on the development of two different categories ofin-situ TiB2-TiN-SiC composite coatings with the incorporation of free hBN and more than stoichiometric amount of SiC without negatively affecting any of its mechanical and tribological properties. To do this, several TiB2-TiN-SiC composites with variable amounts of hBN are fabricated and thoroughly characterized to find out a combination perfectly suitable for engineering applications. Few more TiB2-TiN-SiC samples processed with extra amount of SiC are also tested to achieve a suitable composition with much improved mechanical properties without compromising on tribological properties.
3TiO2 + 2hBN + SiO2 +C = TiB2 + 2TiN + SiC + 4O2
(1)
Experimental procedure Eq. (1) is followed to make a precursor powder mixture containing TiO2, SiO2, hBN and graphite to preplace over surface ground and cleaned AISI1025 steel substrates [size: 100×50×8 mm3] maintaining uniformity in thickness for the preparation of coatings. A highpower CW-fiber laser beam (2 kW laser power) is used in a scanning mode to raise the surface temperature of the samples that initiates a chemical reaction in the preplaced precursor powder mixture. The reaction products of Eq. (1) (titanium-di-boride, titanium
4
nitride, silicon carbide) on subsequent laser treatment, develop a ceramic matrix composite layer with in-situ reinforcements over steel substrates. The composite coating processed with stoichiometric composition according to Eq. (1) is identified as K1. Following the same route, 10 more coatings of different compositions are developed (compositional details are mentioned in Table 1). For samples K2-K8, amount of hBN is increased in the precursor powder mixtures in seven weight increments with respect to sample K1. Similarly, K9-K11 samples are produced with three weight increments of SiC added to the reactant phases in the precursor mixture (TiO2, SiO2, hBN, and graphite powder) with a view to have more SiC than stoichiometric amount in the final coating. Here, the amount of SiC to be added is calculated with respect to the stoichiometric amount of SiC (according to Eq.(1)) that is supposed to be produced in-situ in sample K1. Thorough microstructural studies on developed composite coatings are performed using the scanning electron microscope (SEM) (Zeiss Supra-55and Zeiss EVO 18 Research). Wire-EDM (Electronica-ECOCUT) is used to cut smaller specimens (10×10×8 mm3) from the coated samples for the microstructural studies. X-ray diffractometer (Bruker D2 PHASER) is used under Cu-K radiation to carry out phase analyses of the composite coatings. Also, phases are α
determined with bright field imaging and selected area electron diffraction (SAED) patterns observed in a JEOL JEM 2100 HRTEM (high-resolution transmission electron microscope). Lab-RAMAN Evolution 300 Raman Spectrometer is used for Raman scattering. Tribological properties of composites are evaluated by a linear reciprocating tribometer (DUCOM CM 9104). For the tribological tests, a Wire cut specimen (20×20×8 mm3) from the bulk coated sample is used as a square block against the counterbody. Details of the parameters of the tribological test are enlisted in Table 2. Hardness test has been performed on the cross section of the composite coating using hardness tester ((INNOVA, Falcon 500, Maastricht, The Netherlands). 5
Results and Discussion Microstructural details of the ceramic matrix composite coatings are observed under Scanning electron microscope (Figure 1(a-b)). The magnified views of the representative coating’s cross-section (K2), as shown in Figure 1(a), reveal the distribution of reinforcements in the coating matrix at the cross-section.
SEM images taken in back
scattered emission (BSE) mode indicate towards formation of several phases in the coating. TiB2, TiN and SiC phases (as mentioned in Eq. 1) backscatter differently under the SEM owing to the difference in their average atomic numbers. This results variations in image brightness, which elucidates the distribution of phases in the coating microstructure. EDS analyses showing the presence of Ti, B, N, Si, C and Fe denote dispersion of titanium-rich phases in silicon-rich matrix. The higher magnification of the labeled area of sample K2 shown in the inset of Figure 1(a) portrays a compact and homogeneously distributed microstructure with finer reinforcements. Factors relating to temperature gradient, solidification front velocity and cooling rate play vital roles in determining these microstructural features. Figure 1(b) shows in BSE, the dispersion of white acicular reinforcements in a gray continuous matrix at the cross section of sample K10. Figure 2(a-b) shows bright field image and selected area electron diffraction (SAED) patterns of the composite coating K4 obtained under transmission electron microscope (TEM). The SAED pattern analysis reveals presence of nanocrystalline hBN and TiN in the sample. Diffraction patterns of composite coatings obtained from XRD analyses (Figure 3(a-b)) show the presence of SiC, TiN and TiB2 in all the composite coatings (K1-K11). Traces of hBN are also obtained in samples K2-K8 (Fig 3(a)). Raman spectrum of the sample K1 is obtained over wide range of frequency (up to 450 cm-1) and is presented in Figure 4. The strongest peaks observed at 210, 292 and 343 cm-1 can be associated with the presence of titanium nitride (TiN) which is almost same as those 6
previously reported [25-29]. The low-frequency scattering originates due to transverse and longitudinal acoustical phonons [25, 28]. The presence of TiB2 is confirmed by observing peaks at 271 and 401 cm-1 in Figure 4. These Raman shifts indicating the formation of TiB2 are in good agreement with a previously reported analysis of Raman spectroscopy [28, 30,31]. The present work focuses on the investigation of mechanical and tribological properties of composite coatings developed with varying amounts of hBN as solid lubricant and SiC as wear resistive material in the precursor mixture. The evaluation of the tribological properties of composite coatings K1–K11 has been carried out in a linear reciprocating tribometer at a constant load, under a dry sliding wear condition against WC-6%Co counterbody. The details of the tribological tests performed are mentioned in Table 2. Friction coefficient obtained on addition of different amounts of hBN and SiC in the precursor mixture are presented in Figure 5(a-b). After the initial run-in period, as the quasi-steady state is attained, the values of friction coefficient are noted at all the tests. XRD analysis already confirmed the presence of hBN in composite coatings K2-K8. It is observed from Figure 5(a) that increase in the amount of hBN in precursor causes reduction in friction coefficient only upto K6 and further increment in hBN content results rise in friction coefficient values. Increasing the amount of SiC in coatings beyond stoichiometric composition (K9-K11) also causes rise in friction coefficient (Figure 5(b)). Values of specific wear rate in sliding wear tests conducted are depicted in Figure 6(a-b). Effect of variations in the amount of hBN in precursor mixtures used to fabricate the composite coatings on specific wear rate shows an interesting trend (Figure 6(a)). A small rise observed in specific wear rate values in composite coatings K1–K5 can be corroborated to the increasing amount of hBN in precursor and beyond certain percentage weight addition of hBN, values of specific wear rate (K6-K8) increase even faster. Increase in the amount of
7
SiC in the composite coatings (K9-K11) causes a gradual reduction in specific wear rate values (Figure 6(b)). Figure 7 represents cross-sectional microhardness of all composite coatings developed in the present study. It reveals that with the rise in hBN content up to certain level (between K2 and K5), minor reduction in hardness values are observed. Beyond this, composite coatings K6K8 with even higher amount of hBN, show reduction in hardness values at significantly faster rate. Similarly, additional amount of SiC in composite coatings K9 and K10, causes an increment in hardness values. With even higher amount of SiC, K11 display decrease in the hardness value. Friction coefficient in a situation of sliding wear is influenced by many factors and is evaluated by summing up the contributions of adhesion, elasto-plastic deformation and the presence of tribo-films respectively [32-33]. For composites, tribological behavior is affected significantly by the participation of wear debris in the interaction with a counterbody and environmental effects such as temperature and humidity [34]. However, in the present study the tribological tests are conducted in constant temperature and humidity and hence the effects are uniform to all the samples. From the XRD analyses, presence of highly-ordered crystalline hBN in coated samples K2K8 is confirmed. This corroborates to detection of an intense peak at 26.6° (2θ) of the diffraction data, assigned to the crystallographic plane (002) of hBN [35]. Similar peak is observed at 2θ ∼ 26° of the hexagonal graphitic structure [36]. A lamellar structure similar to that of graphite is present in hBN with boron and nitrogen atoms having covalent bonds in between, forming hexagonal B3N3 cells and two-dimensional BN layers stacked with weak van der Waals forces [37]. The spacing (0.34 nm) in between these layers is very much similar to that in graphite (0.335 nm). This lamellar structure of hBN is weak in shear and display lubrication properties in a tribological interaction. In this study, presence of the hBN 8
in the composite coatings K2-K8 caused reduction in friction coefficient as compared to the composite coatings K1 and K9-K11. Interestingly, increasing amount of hBN shows a decreasing trend of friction coefficient within K2-K6 and on further increase in hBN content in K7-K8; the trend seems to have reversed with the rise in friction coefficient values. Here, the presence of certain higher amount of hBN resulted in the weakening of bond strength between the coating matrix and reinforcements which may overweigh the solid lubricant effect and nucleate more cracks along with deep grooves in the composite coating resulting easier dislodgement of particles [38]. At the zone of tribological interaction, dislodged particles of coatings trapped between test sample and counterbody result in a significant rise in friction coefficient. Worn surface morphologies observed under SEM display higher amount wear debris along with some fracture pits on sample K7 and K8. A composite, containing higher percentage of softer phases like hBN in its matrix, on being subjected to a higher contact pressure at the zone of tribological contact in a sliding wear test produce gross surface and subsurface deformation and cracking. As a result, generation of debris and subsequent formation of fracture pits of considerably larger dimensions are observed. The sample surfaces (K7-K8) get repeatedly grounded by wear debris forming fresh surfaces of interaction and the worn surfaces show severe damage. In such conditions, wear mechanism is observed to be dominated by abrasive wear and higher values of friction coefficient are recorded [39]. Crystallographic orientation and other bulk properties are found to influence material’s frictional response. Narrower dislocations are formed due to the presence of a significantly high amount of hard SiC particles with covalent bond. This leads to the increase in friction coefficient of composite coatings K9-K11. Values of specific wear rate of composite coatings K2-K5 show minor increase because of the presence of hBN. Soft and lubricious hBN in the coatings may reduce brittle fracture remarkably and result a smoother worn surface after the sliding wear test. The mode of
9
fracture in composites change from trans-granular to inter-granular due to the presence of hBN causing the fracture toughness to rise. The deflection of the crack at the hBN interface can explain such improvement [38]. A tougher matrix of composite reduces crack nucleation, which eventually helps in improving its wear resistance properties. Increment in hBN amount further to the sample K5, as in samples K6 and K7, shows a significant rise in specific wear rate. Presence of certain higher amount of hBN in composites leads to weakening of matrix and results greater loss of material under the dynamic loading condition of tribological interactions. Here, the softer hBN, upon agglomeration, tends to form zones of continuous hBN phase, augmenting nucleation and propagation of tensile cracks during tribological interactions [13]. This causes the composite to lose the tribological advantage of having hBN in its matrix [38]. Microhardness is a key factor in establishing composites’ tribological properties. Coated samples K2-K5 are observed to have similar hardness values and on further increase in hBN amount (in K6-K8) significant degradation in hardness is observed. On the other hand, modification in grain boundary strength and reduction in the surface damage associated with intergranular fracture are observed due to the presence of higher amount of SiC in composites [40]. Movement of narrower dislocations formed in the coating matrix in presence of SiC occurs only under larger values of stresses [23]. Asl et al. noticed that improvement in density and microstructural characteristics are obtained with higher amount of SiC in composites [41]. All these result in wear resistance to improve significantly in composites developed with the excess amount of SiC in precursor. Figure 8 shows wear track morphologies of composite coatings K1-K11 generated in sliding wear tests. The worn-out surfaces on samples K1-K5 suggest domination of adhesive wear with occurrences of delamination happening in presence of both single and repeated cycle deformation mechanisms under the cyclic stresses of tribological tests. Visible signs of
10
abrasive wear on K1-K5 are lesser as compared to that in samples K6-K8. Smaller quantities of debris formed during tribological interactions augment abrasive wear. At higher hBN content, degradation of mechanical properties of ceramic composites leads to spallation and pitting. The wear debris retaining at the wear track get accumulated in these surface defects and weaken the effect of abrasive wear. Wear tracks on composite coatings K6-K8 show formation of deeper grooves, larger pits and wider cracks, which correspond to higher specific wear rate and corroborates the disadvantages imparted by the presence of higher amount of hBN in the coating matrices. Dislodgement at the coated surface and resultant material losses are much less abrupt in the composite coatings K9-K11, fabricated with higher amount of SiC. Surface deformation and damage at smaller scales make the surfaces of tribological interactions appear relatively smoother as compared to other coatings. Conclusion: A precursor powder mixture, prepared following the stoichiometry of a high-temperature chemical reaction, is preplaced over a steel substrate and subsequently laser treated to develop an in-situ TiB2-TiN-SiC composite coating. Seven more compositions of precursors with more than stoichiometric amount of hBN are used to produce TiB2-TiN-SiC-hBN coatings with varying weight percentages of hBN. Similar method is used to fabricate TiB2TiN-SiC coatings with variable amounts of SiC. SEM studies portray the composite coatings’ compact and homogeneously distributed microstructures with finer reinforcements developed without any form of discontinuities. XRD analyses show the presence of all expected phases in composite coatings developed with the combination of high-temperature chemical reaction and laser treatment. The stoichiometric composition of in-situ TiB2-TiN-SiC composite coating appears to be good in terms of hardness and tribological properties. Enhancement in friction and wear behaviors can be correlated to the presence of free-hBN in composite matrices. Remarkable changes in mechanical and tribological properties are noticed with the 11
increase in hBN content. Interestingly, advantages are obtained only up to a certain weight percentage of addition, and beyond that degradation in friction and wear resistance is observed. In the present work, improvement in frictional properties is achieved on addition of hBN up to K6 composite coating. The lamellar structure of hBN (sp2 hybridization) is weak in shear and displays lubrication properties in tribological interactions. In K7-K8, the trend seems to have reversed. Here significant rise in the values of friction coefficient is observed because of the presence of certain higher amount of hBN. This resulted in the weakening of bond strength between the coating matrix and reinforcements which overweighs the effect of solid lubrication, resulting in easier dislodgement of particles. Dislodged particles of coatings trapped between test sample and counterbody also contribute to rise in friction coefficient. Addition of free-hBN in composite coating K2-K5 shows minor change in specific wear rate with small change in hardness. This tribological advantage of solid lubrication is lost in samples K7 and K8 because of the presence of certain higher amount of hBN in composites leading towards weakening of matrix. This results in greater loss of material under the dynamic loading condition of tribological interactions. Similarly, increasing the SiC content in TiB2-TiN-SiC matrix beyond the stoichiometric composition affects the hardness and frictional properties. In this study, coatings K9-K10 show significant improvement in hardness and wear resistance and further addition of SiC shows reversal of the trends in the hardness of the composite. This study presents a wholesome idea of the property tailoring of TiB2-TiN-SiC composites affected by hBN and SiC. Acknowledgments The authors are thankful to Sophisticated Instrument Centre (SIC) and Discipline of Physics of IIT Indore. Facilities received from Department of Science and Technology, Government. of India, under FIST scheme (grant number SR/FST/PSI-225/2016) is highly acknowledged. Authors are grateful to Central Workshop and AMP Laboratory of IIT Indore, for material 12
processing and fabrication works. The support received from Dr. I. A. Palani, Head, Discipline of Mechanical Engineering, IIT Indore is gratefully acknowledged. The authors thank members of the Laser Processing Laboratory of Department of Mechanical Engineering and Central Research Facility (CRF) of IIT Kharagpur, for their involvement in the work. Financial support from DST-SERB and MHRD are gratefully acknowledged. Reference [1] K. Miyoshi, Solid liubrication Fundamentals and Applications, Kazuhisa Miyoshi, New York (2001). https://doi.org/10.1201/9780429134333 [2] Francis J. Clauss, Solid Lubricants and Self-Lubricating Solids, Academic press, New York (1972). [3] S. Mazumder, O.P. Kumar, D. K. Kotnees, N. Mandal, Tribological Influences of CuO Into 3Y-TZP Ceramic Composite in Conformal Contact. J. Tribol. 141(2019),031606. https://doi.org/10.1115/1.4041894 [4] L. Du, W. Zhang, W. Liu, J. Zhang, Preparation and characterization of plasma sprayed Ni3Al–hBN composite coating, Surf. Coat. Technol.205(2010), 2419-2424. https://doi.org/10.1016/j.surfcoat.2010.09.036 [5] Y. Sun, Q. Meng, D. Jia, C. Guan, Effect of hexagonal BN on the microstructure and mechanical properties of Si3N4 ceramics, J. Mater. Process. Technol. 182(2007) 134-138. https://doi.org/10.1016/j.jmatprotec.2006.07.020 [6] X.L. Lu, X. B Liu, P. C Yu , S.J. Qiao, Y.J. Zhai, M. D. Wang , Y. Chen, D. Xu, Synthesis and characterization of Ni60-hBN high temperature self-lubricating anti-wear
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List of Table Caption Table 1: Details of ceramic matrix composite coatings developed. Table 2: Operating conditions for tribological characterization
List of Figure Caption Figure 1: SEM images of cross-sections of representative composite coatings (a) K2 and (b) K10 (magnified view of the labeled area on the cross-sectional view of K2 is given in the inset). Figure 2: TEM image of K4 sample showing the (a) top surface morphology and corresponding (b) Selected area electron diffraction (SAED). Figure 3: X-Ray diffraction spectra of composite coatings. (a) K1-K8 and (b) K9-K11. Figure 4: Raman spectrum of sample K1 showing the presence of TiN and TiB2 phases. Figure 5: Variations in friction coefficients of laser surface alloyed composite coatings developed with compositional variations (a) K1-K8 and (b) K1, K9-K11 Figure 6: Variations in specific wear rate of different composite coatings developed with compositional variations (a) K1-K8 and (b) K1, K9-K11. Figure 7: Vickers Hardness of different composite coatings developed with compositional variations. Figure 8: SEM images showing morphologies of wear tracks formed on composite coatings K1-K11 at 12.5 N normal load.
20
Table 1: Details of ceramic matrix composite coatings developed Samples Nomenclature
K1 K2 K3 K4
CMC coatings deposited on steel substrate
Composition of pre-placed powder layer TiO2 SiO2 C hBN SiC (g) (g) (g) (g) (g) TiB2+TiN+SiC 10 2.5 0.5 2.06 --Additional wt.% hBN in precursor 2.36 --2.68 ----3.02
K5
3.54
---
4.1
---
K7
4.8
---
K8
5.36
---
K6
TiB2+TiN+SiC+hBN
10
2.5
0.5
Additional wt.% SiC in precursor 0.76
K9 K10 K11
TiB2+TiN+SiC
10
21
2.5
0.5
2.06
1.84 2.96
Table 2: Operating conditions for tribological characterization Load
Stroke
Frequency
WC-Co Ball diameter
Test time
(N)
(mm)
(Hz)
(mm)
(Min)
12.5
3
20
6
15
22
Figure 1: SEM images of cross-sections of representative composite coatings (a) K2 and (b) K10 (magnified view of the labeled area on the cross-sectional view of K2 is given in the inset).
23
TiN
hBN
TiN hBN
(a)
20nm
(b)
Figure 2: TEM image of K4 sample showing the (a) top surface morphology and corresponding (b) Selected area electron diffraction (SAED).
.
24
-TiN
- SiC
- TiB2
Cu-Kα
- hBN
(a)
-TiN
K8
- SiC
- TiB2
Cu-Kα
(b)
Intensity (a. u.)
Intensity (a. u.)
K7 K6 K5 K4 K3
K11
K10
K2 K1 20
30
40
50
60
70
K9 20
80
30
40
50
60
70
80
2θ (degree)
2θ (degree)
Figure 3: X-Ray diffraction spectra of composite coatings. (a) K1-K8 and (b) K9-K11.
25
Intensity/Counts
Fit Peak 1 Fit Peak 2 Fit Peak 3 Fit Peak 4 Fit Peak 5 Cumulative Fit Peak Data
150
200
250
300
350
400
450
Raman shift (cm-1)
Figure 4: Raman spectrum of sample K1 showing the presence of TiN and TiB2 phases.
26
1.0 0.9
1.0
(a)
Stroke length: 3 mm
0.7 0.6 0.5 0.4 0.3 0.2 K1
K2
K3
K4
K5
K6
K7
K8
0.0
Test load: 12.5 N
(b)
Stroke length: 3 mm
0.8
Friction coefficient
Friction coefficient
0.8
0.1
0.9
Test load: 12.5 N
0.7 0.6 0.5 0.4 0.3 0.2 0.1
K1
K9
K10
K11
0.0
Composite coating
Composite coating
Figure 5: Variations in friction coefficients of laser surface alloyed composite coatings developed with compositional variations (a) K1-K8 and (b) K1, K9-K11
27
6
Specific wear rate (10-5mm3/N-m)
Specific wear rate (10-5mm3/N-m)
7
Test load: 12.5 N
(a)
Stroke length: 3 mm Frquency: 20 HZ Ball: WC-Co (6mm)
5 4 3 2 1 K1
K2
K3
K4
K5
K6
K7
K8
0
Composite coatings
4.0 3.5
Test load: 12.5 N
(b)
Stroke length: 3 mm Frquency: 20 HZ
3.0
Ball: WC-Co (6mm)
2.5 2.0 1.5 1.0 0.5 K1
0.0
K9
K10
K11
Composite coatings
Figure 6: Variations in specific wear rate of different composite coatings developed with compositional variations (a) K1-K8 and (b) K1, K9-K11
28
Vickers Hardness (HV0.10)
2400 2200 2000 1800 1600 1400 1200 1000
K1
K2
K3
K4
K5
K6
K7
K8
K9
K10
K11
Composite Coatings
Figure 7: Vickers Hardness of different composite coatings developed with compositional variations.
29
K2
K1
Debris
Scratch Pit
Delamination
20 µm
20 µm
K3
Debris Pit
Delamination
20 µm
Scratch
K4
K5
Debris
Pit
K7 Spallation
Pit
20 µm K9
K8 Pit
Pit Debris
Spallation Debris
20 µm Groove
Spallation
20 µm
20 µm
Debris
K6
Scratch
Spallation
20 µm
Micro cracks
K10
20 µm
K11 Pit
Delamination Scratch
Scratch
Debris
20 µm
20 µm
Figure 8: SEM images showing morphologies of wear tracks formed on composite coatings K1-K11 at 12.5 N normal load
30
To The Editor Ceramic International Dear Sir, Please find the attached manuscript titled “Effect of hBN and SiC Addition on Laser Assisted Processing of Ceramic Matrix Composite Coatings” for the consideration for publication in the ‘Ceramic International’. On the behalf of the authors, I hereby confirm that there are no known conflicts of interest associated with this publication.We also confirm that our work is original and has neither been published elsewhere nor is currently under consideration for publication in any other place. Thank you Sincerely,
Satyajit Chatterjee Corresponding Author Associate Professor Discipline of Mechanical Engineering Indian Institute of Technology Indore, India Email: [email protected]