Microstructural characteristics and mechanical properties of thermally sprayed conventional ceramic coatings reinforced with multiwalled carbon nanotubes

Abstract

This paper aims to develop a high-performance, cost-effective ceramic coating on AISI 1020 steel substrate. For this purpose, three kinds of conventional ceramic powders are taken in the composition Al₂O₃+3 wt%TiO₂, WC+12 wt%Co and ZrO₂+8 wt%Y₂O₃. The composites are coated on the steel surface using thermal spraying technology such as plasma flame for Al₂O₃+3 wt%TiO₂ and ZrO₂+8 wt%Y₂O₃ and high-velocity oxy-fuel for WC+12 wt%Co deposition. Recently, carbon nanotube–reinforced ceramic materials have gained more attention because of their excellent surface morphology properties. Adding nanocomposite to ceramic powders can considerably increase the microstructural characteristics and microhardness of thermal sprayed coatings. However, it is still challenging to obtain effective nanocomposite-doped conventional coating due to the elevated temperature of heat equipment. Thus, the conventional ceramic powders Al₂O₃+3 wt%TiO₂, WC+12 wt%Co and ZrO₂+8 wt%Y₂O₃ are combined with carbon nanotubes at a weight ratio of 1%, 3% and 5% for preparing reliable nanocomposite ceramic coating. The properties of nanocomposite mixed coatings are studied, and their performance is compared with conventional coatings. The coatings’ morphology, structure and phase composition are investigated through scanning electron microscopy and X-ray diffraction. In addition, Vickers microhardness and surface roughness are also determined.

Keywords

Plasma spray high-velocity oxy-fuel carbon nanotubes scanning electron microscopy x-ray diffraction surface roughness microhardness

Introduction

In thermal applications, the metallic steel substrate, alloy components and high temperature–exposed heat engines must be protected from friction and wear damages like corrosion, erosion, adhesion and abrasion.^1,2 Surface modification technology has been introduced to provide resistivity against these issues owing to material degradation in the future. Because of their remarkable properties, thermal barrier coatings (TBCs) are widely used to protect equipment in various applications, including aerospace, aviation, weapons and warships,^3,4,5 to increase the lifetime efficiency of engines by protecting them from excessive temperature. However, existing works have been limited by a number of disadvantages such as poor wear resistance, low surface hardness and so on.^6,7 Numerous approaches are available to deposit TBC coating over a metallic substrate which is to be protected from heat and corrosion. High-temperature thermal spraying is the extensively used coating deposition technique, further classified as air plasma spraying (APS), detonation gun spraying and high-velocity oxy-fuel spraying (HVOF). The primary goal of these techniques is to provide coatings with splat boundaries and pores as it lessens the global temperature of the surface. It can also reduce the microstructural properties, which improves the strain compliance of rigid ceramic on a high coefficient thermal expansion (CTE) substrate.

Generally, in most of the applications, both alumina and titania wear-resistive powders are taken as feedstock for plasma spraying.⁸ The alumina and titania composites are well known for their low thermal expansion, low thermal conductivity and high toughness.⁹ Sabiruddin et al.¹⁰ reported the maximal hardness of alumina coating as 1200 HV with porosity in the range 5–9.4%. TiO₂ is identified as an effective anti-corrosion material because of its large chemical stability, low electron conductivity and temperature resistance. Lima and Marple¹¹ identified the hardness and porosity of titania coatings as 850 ± 50 HV and 2.5 ± 0.4%, respectively. Moreover, the fracture toughness and adhesion strength of ceramic coating are improved after mixing the compound titania with Al₂O₃. For the past few decades, several processes have been determined to enhance YSZ coatings’ durability since it is affected by the premature failure caused by residual stress, microcrack formation, extension and coupling.^12,13,14 Coatings made from ZrO₂ ceramic powders extend the durability by lowering the thermal stress with low thermal expansion coefficients. The 8 wt% Y₂O₃ is found to be an optimum mixture for substrates when it is subjected to high temperature.¹⁵ Furthermore, the stabilized oxides in the mixture of Y₂O₃ and ZrO₂ will stop the formation of oxide substances in the layers of the substrate.¹⁶ WC-Co is considered one of the best options for increasing the wear resistance of surface components due to its high hardness, chemical inertia and elasticity.^17,18,19,20 However, the application of WC-Co is limited to a temperature below 450°C.²¹ Since little attention has been paid to the performance improvement to date, it is necessary to consider the oxidation and wear resistance in a high-temperature environment.

Nanocomposite plays a significant role in the progress of TBC as stated in Ref. 22. Multiwalled carbon nanotubes (MWCNTs) are recently used as an effective reinforcement material that can be incorporated with conventional ceramic powders to form composites with superior properties. CNTs exhibit high thermal conductivity with elasticity and tensile strength in the range 1000 GPa and 11–63 GPa, respectively.^23,24 For instance, 0.07 wt% CNTs doped in ceramic alumina and fabricated by a hot pressing process provides a 50% fall in dry sliding wear proportion, including fracture toughness and improved hardness of 27% and 9%, respectively.²⁵ Balani et al.²⁶ identified that CNT-incorporated thermal sprayed coating has a higher hardness of 739 HV, whereas coating without CNTs has a hardness of 714 HV. Besides, the application of multiwalled CNTs is restricted by the following two issues: Firstly, lack of knowledge to get a homogeneous distribution of matrix composite and secondly, improper interface between the ceramic substrate and the MW-CNTs. These issues cause CNTs agglomeration in the final composite, leading to premature failure of the substrate.

With all these perspectives in mind, the present work is implemented to gain multiwalled CNT–reinforced ceramic composite with good mechanical strength and thermal conductivity. The main objective of this paper is to investigate and analyse the effectiveness of plasma flame and HVOF-sprayed conventional as well as the CNT-doped ceramic coatings in metallic substrate exposed to high temperature for a longer time. The stability of the conventional ceramic coating is identified and compared with the corresponding CNT-doped ceramic coating. For this purpose, three different compositions of conventional powders Al₂O₃+3 wt%TiO₂, WC+12 wt%Co and ZrO₂+8 wt%Y₂O₃ are taken as topcoat and Ni-5%Al or Ni-5%Cr are considered as a bond coat. Moreover, for comparative analysis, each conventional powder is doped with CNTs in the ratios 1%, 3% and 5%. The ceramic powders Al₂O₃+3 wt%TiO₂ and ZrO₂+8 wt%Y₂O₃ are deposited with plasma spraying, whereas WC+12wt%Co is deposited with high-velocity oxy-fuel spraying. The characteristics of coatings are evaluated in terms of surface hardness, surface roughness, XRD assessment and SEM pattern analysis.

Experimental details

During this experiment, three conventional ceramic powders are taken as topcoat in the composition: Al₂O₃+3 wt%TiO₂, WC+12 wt%Co and ZrO₂+8 wt%Y₂O₃. These powders were purchased from Metallizing Equipment Co. Pvt. Ltd., Jodhpur. The MWCNTs produced by chemical vapour decomposition with an average outer diameter of 24 nm and average length 10 μm are taken as a reinforcement material. The multiwalled CNTs’ specific surface area and bulk density are 220 m²/g and 0.14 g/cm³, respectively. The purity of the nanocomposite, as reported by the manufacturer, is > 98%. The conventional ceramic powders are incorporated with CNTs through a dry mixing process called ball milling. Al₂O₃+3 wt%TiO₂, WC+12 wt%Co and ZrO₂+8 wt%Y₂O₃ mixtures are subjected to ball milling for 8 h to integrate carbon nanotubes in the ratio 1%, 3% and 5%. This will produce nine different CNT-reinforced mixtures with three compositions from each of the conventional powders. Hardened steel balls or tungsten carbide balls are generally used in the blending process for powder ratio of 10:1 for all the mixtures. The ball milling is done with a rotation speed of 100 r/min. The process is stopped for 30 min after every 1 h of milling to manage the overheating of powders.

AISI 1020 steel samples of size 65 × 55 × 5 mm are cut from the bars with width and thickness, respectively, in the range of 55 mm and 5 mm using a power hacksaw. To eliminate oxide formation, the top and bottom surface of the substrate is ground by a surface grinding machine (Alex NH 500). It introduces a flat surface substrate with roughness (Ra) around 0.1 μm. Before thermal spraying, the substrate is grit blasted within a suction-type cabinet specially made for grit-blasting purpose with alumina grits of mesh size 24, under air pressure 100 lbf/in² and at a standard-off distance of 125 mm. This will provide proper mechanical anchorage between the substrate and the bond coat by increasing the roughness of the substrate to 5 μm Ra. The oxide layer formed from the time gap between surface grinding and grit blasting can be removed at the end of grit blasting. Afterwards, the grit-blasted substrate is subjected to ultrasonic cleansing in iso-propanol for 10–15 min to get rid of dirt, silica powders and other foreign particles present in it. Before depositing the bond coat, the prepared substrate is pre-heated to 200°C with a plasma gun filled with nitrogen as plasma gas. This pre-heating will eliminate the substances such as oil, grease and water vapour and then present a neat and nascent surface for bond coat deposition.

The plasma spraying technique is followed to deposit Al₂O₃+3 wtTiO₂ and ZrO₂+8 wt%Y₂O₃ coatings using a plasma gun (SulzerMetco 3 MB) placed on the CNC XY table. The plasma gun contains nitrogen as the primary gas and hydrogen as the secondary gas. Nitrogen also acts as the carrier gas for transferring the ceramic powder from the powder feeder to the plasma gun. The process parameters of these powders show a considerable impact on the final voltage and arc power per rate of flow of nitrogen. The plasma gas is sprayed on all types of coating at an angle of 90°. The surface of the substrate is cooled on coating deposition due to the parallel alignment of two auxiliary jets towards the plasma flame. This will eliminate unmelted or imperfectly bonded powder impurities from the surface of the substrate. Before the deposition of a ceramic composite mixture, the substrate is coated with Ni–20 wt%Cr as a bond coat of width 100–150 μm. This bond coating will increase the topcoat adhesion and decrease the thermal expansion coefficient mismatch between substrate metal and composite coating. This process is further repeated to deposit 1%, 3% and 5% CNT-doped mixtures of Al₂O₃+3 wtTiO₂ and ZrO₂+8 wt%Y₂O₃ powders. High-velocity oxy-fuel spraying with an HP-700 spraying gun is used to prepare WC+12 wt%Co conventional powder. The conventional and CNT-reinforced tungsten cobalt powder coatings are deposited separately on 130 × 25 × 5 mm–sized AISI 1020 steel substrate using a thermal spraying approach. During this process, propane and nitrogen play a major role of fuel gas and carrier gas, respectively. The oxygen-to-fuel volume ratio is taken as 3.4–3.8 for coating deposition from a spraying distance of 150–200 mm.

The surface morphology of the ceramic powders and coated substrate are analysed using a Zeiss scanning electron microscope (SEM). The cross-sections of as-prepared coatings are cut to 10 × 5 × 5 mm size for SEM analysis. The qualitative analysis of phase constitution available in a coated substrate is examined with PAN analytical X’pert PRO PW1070 X-ray diffractometer with Cu-Kα radiation operating under 30 mA intensity and 40 kV tension. The scanning step to the step time of the diffractometer is 0.016711 and 0.13 s, respectively. The data obtained from XRD is then analysed using software named X’Pert High Score. The phase distribution of the coating is quantified with the rate of relative phase intensity as stated in Ref. 27. The surface roughness of the coated substrate is estimated using a surface roughness tester, Taylor Hobson precision surtronic 3+ profilometer. The surface hardness of the coated substrates is determined using the LECO LM 700 microhardness tester mounted on a polished surface with Vickers indenter at 100 g load and dwell time of 15 s. Five readings are taken on each coating during surface roughness and surface hardness measurement to calculate the mean and standard deviation values.

Results and discussion

The SEM micrograph in Figures 1(a)–(c) shows the size of three blended ceramic powders. Moreover, it reveals the presence of pores between the three blended micron powders. The addition of CNT in different wt% with Al₂O₃-3%TiO₂ minimizes the formation of the pores as shown in Figures 2(a)–(c). A similar observation was noticed in Ref. 28. Figures 3(a)–(c) exhibit SEM images of CNT-added ZrO₂-8%Y₂O₃ ceramic powder. The ceramic feedstock powder of ZrO₂-8%Y₂O₃ is broken during the CNTs mixed with ZrO₂-8%Y₂O₃ and the influence of CNTs on the mechanical blending of the particle crushed. The increasing wt% of CNTs fills the gaps between the two-micron–sized ceramic powders, and the same observation is seen in Figures 4(a)–(c) of WC-12%Co ceramic powders.

Figure 5 presents the SEM images of coated samples. The pores’ occurrence and adhering of ceramic particles are elucidated in Figure 5(a). These results imply that more pore formation may be due to the partial melting of powders. The CNT’s presence and its bonding with conventional ceramic powder are shown in Figure 5(b). Figure 5(c) revealed that all the ceramic particles are melted completely with the clustering of CNTs. The CNT’s presence is shown in Figure 5(d). Further, porosity formations are observed, and the image notices no cracks. Figure 5(e) and (f) show the images of WC+12%Co+5% CNTs’ composite coating. The cobalt is a binder that completely adheres to the WC. The conventional and CNT particles are completely diffused and distributed uniformly. The non-uniform melting and distribution of CNTs in coating increases the porosity.²⁹ The agglomerates of CNTs in the coating may create non uniformity in CNTs melting, resulting in more porosity. The nominal inclusion of CNTs supported the alumina in retaining the liquid state for a long time, which in turn might develop good distribution of alumina in CNT-reinforced coating.³⁰

Figure 6 shows the SEM images of the cross-sectional thickness of the coatings obtained with 5 wt% of CNTs reinforcement on different base materials of Al₂O₃+3%TiO₂, ZrO₂+8%Y₂O_3, and WC+12%Co. The coating thickness of substrate materials is 474 μm, 470 μm and 495 μm. This coating completely agglomerates to the surface, in the form of metallurgical bonding.

X-ray diffraction analysis

Figure 7 shows the XRD pattern of Al₂O₃-3%TiO₂ and CNT-added composite coating. The results confirmed the presence of Al₂O₃, TiO₂ and CNTs. There is no evidence of the creation of a new compound in the coating. The ZrO₂-8%Y₂O₃ and the CNTs’ presence in the coating are identified by the XRD pattern given in Figure 8. The data indicated that no compounds were formed in the coating, but the elements ZrO₂, Y₂O₃ and CNTs are present in the coating. Figure 9 represents the XRD pattern of WC-12%Co and the CNT-added coating. It is noticed that only WC, Co and CNT elements are available, and no compounds are formed in the coating.

Microhardness of coating

Figure 10 shows the results of microhardness of conventional and CNT-added composite coatings. An average of three measurements is taken for the study. The results indicated that the addition of CNTs improved the microhardness of the coating due to grain refinement.³¹ The higher inclusion of CNTs in the composites may form clustering, which reduces the hardness of the coating. The microhardness of Al₂O₃-3%TiO₂-3% CNTs exhibited higher microhardness rather than other compositions. The increase of CNTs above 3wt% in the Al₂O₃-3%TiO₂ coating reduces the hardness of the coating, and this may be due to the agglomeration of CNT in the coating. ZrO₂-8%Y₂O₃ composition shows lower microhardness. WC-12%Co +1% CNTs composite coating has better microhardness in that series. The marginal variation in hardness was observed in the CNT-added WC-12%Co coating.

Table 1 shows the statistical results for the microhardness of composite coating. The overall microhardness value for all upper and lower quartile composite coatings falls between 438.01 HV and 876.55 HV. These values determine the distribution for the measure of the mean. Among the three different coatings, WC-12%Co coating with and without CNTs has a minimum spread between the lower quartile value of 536.35 HV and the higher quartile value of 604.66 HV. These exhibited that all elements were mixed homogeneously throughout the coating and bonded very well with the substrate.

Table 1.

Statistical analysis for microhardness of composite coating.

Sample	Al₂O₃-3%TiO₂				WC-12%Co				ZrO₂-8%Y₂O₃
Sample	0% CNT	1% CNT	3% CNT	5% CNT	0% CNT	1% CNT	3% CNT	5% CNT	0% CNT	1% CNT	3% CNT	5% CNT
SD	8.86	7.29	5.91	5.39	4.46	4.78	2.71	5.18	3.3	2.56	1.24	1.2
CV	1.24	0.9	0.68	0.67	0.83	0.8	0.4	0.92	0.75	0.52	0.38	0.21
Mean	715.25	811.94	870.53	802.4	539.46	599.76	574.38	562.21	440.81	493.19	507.35	566.72
Lower quartile	707.77	805.43	865.1	797.21	536.35	595.26	572.24	556.67	438.01	490.41	506.11	565.68
Upper quartile	724.81	819.01	876.55	806.33	543.43	604.66	577.29	567.27	445.26	495.38	508.7	567.65
Median	714.13	811.87	869.61	802.68	540.31	599.67	573.22	562.46	440.27	493.38	509.65	568.72

Surface roughness

Figure 11 represents the surface roughness of conventional and CNT-added composite coatings. The conventional ZrO₂-8%Y₂O₃ revealed higher surface roughness than other conventional composite coatings. However, increasing wt% of CNTs to a certain range reduces the surface roughness of the coating, and beyond it, the roughness is increased. The 1 wt% of CNTs in Al₂O₃-3%TiO₂ and ZrO₂-8%Y₂O₃ shows nominal surface roughness. The 5 wt% of CNTs in WC-12%Co seems to show the lowest surface roughness among all other compositions. The micrograph of WC-12%Co summarized that melting of all elements and uniform distribution of particles enhanced the performance of the coating. In all coatings, WC-12%Co coatings were ductile, resulting in minimum surface roughness.

Figure 12 represents the profile of surface roughness obtained for Al₂O₃-3%TiO₂ with 1%, 3% and 5% CNTs’ coating. The profile shows that surface roughness depth is small for Al₂O₃-3%TiO₂ with 1% CNT-coated sample, as shown in Figure 12(a) when compared with other profiles. These reflected that low roughness value and number of high peaks attained are minimum. The surface profile for WC-12%Co with 1% CNTs is represented in Figure 13. Figure 13(c) has shown low peaks with smaller peak width, resulting in a better surface than other compositions. Figure 14 exhibits the surface roughness profile of ZrO₂-8%Y₂O₃ with 1%, 3% and 5% CNTs’ coating. The peak width and height are the highest in Figure 14(c) than the other two coatings. These coating reflected more roughness value. From these, it is evident that the uniform melting of all elements minimizes the roughness of the coating surface.

Conclusions

The nanocomposite-incorporated ceramic coating for highly corrosive metallic surfaces is prepared in this research with CNTs under three distinct compositions like Al₂O₃ + 3 wt%TiO₂, WC + 12 wt%Co and ZrO₂ + 8 wt%Y₂O₃. The coated surface characteristics, microhardness and surface roughness were analysed in this present work. Based on this investigation, the following conclusions were drawn.

It is observed that partial melting of ceramic powders maximizes the pores, and hence, increasing the wt% of CNTs will decrease the pore formation to a significant range.

XRD patterns of coatings confirmed the presence of composites, and proved that there is no new compound formed on the surface.

CNTs’ agglomeration has reduced the microhardness; however, CNTs’ doping increased the microhardness due to proper grain refinement of powder particles.

Improved surface roughness is also achieved by increasing the CNT content to a certain limit; Nevertheless, in some cases, it is found to be nominal due to semi-molten particles.

The coating performances showed that adding CNTs to a certain wt% enhanced the microhardness and surface roughness. It will provide excellent mechanical properties and withstand corrosion with uniform microstructural characteristics. It is concluded that Al₂O₃-3%TiO₂-3% CNTs coating has higher microhardness and nominal surface roughness.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Venkateshwarlu Bolleddu

References

Hou

, et al. Preparation of plasma sprayed mullite coating on stainless steel substrate and investigation of its environmental dependence of friction and wear behavior. Trib Int 2015; 91: 32–39. DOI: 10.1016/j.triboint.2015.06.022.

Zavareh

Sarhan

Razak

, et al. Plasma thermal spray of ceramic oxide coating on carbon steel with enhanced wear and corrosion resistance for oil and gas applications. Cera Int 2014; 40(9): 14267–14277. DOI: 10.1016/j.ceramint.2014.06.017.

Loghman-Estarki

Razavi

Jamali

, et al. Effect of scandia content on the thermal shock behavior of SYSZ thermal sprayed barrier coatings. Cera Int 2016; 42(9): 11118–11125. DOI: 10.1016/j.ceramint.2016.04.018.

Krella

Sobczyk

Krupa

, et al. Thermal resistance of Al2O3 coating produced by electrostatic spray deposition method. Mech Mater 2016; 98: 120–133. DOI: 10.1016/j.mechmat.2016.05.002.

Kotlan

Seshadri

Sampath

, et al, On the dielectric strengths of atmospheric plasma sprayed Al2O3, Y2O3, ZrO2–7% Y2O3 and (Ba, Sr) TiO3 coatings. Cera Int 2015; 41(9): 11169–11176. DOI: 10.1016/j.ceramint.2015.05.066.

Zhang

, et al. A high strength and good ductility Mg–Y–NI–TI alloy with long period stacking ordered structure processed by hot rolling and aging treatment. Mater Sci Eng:A 2015; 648: 134–139. DOI: 10.1016/j.msea.2015.09.057.

Rolink

Weisheit

Biermann

, et al. Investigations of laser clad, thermal sprayed and laser remelted AlSi20-coatings on magnesium alloy AZ31B under constant and cycling thermal load. Surf Coat Tech 2014; 259: 751–758. DOI: 10.1016/j.surfcoat.2014.09.049.

Bolleddu

Racherla

Bandyopadhyay

. Microstructural and tribological characteristics of air plasma sprayed Alumina-Titania coatings. Production, Properties, and Applications of High Temperature Coatings. USA: IGI Global PublicationsIGI Global, 2018, pp. 268–298, doi:10.4018/978-1-5225-4194-3.ch011.

Kovaleva

Prozorova

Arseenko

, et al. Deposition and characterization of alumina–titania coating by multi-chamber gas-dynamic sprayer. Res Phys 2015; 5: 1–2. DOI: 10.1016/j.rinp.2014.12.003.

10.

Sabiruddin

Joardar

Bandyopadhyay

. Analysis of phase transformation in plasma sprayed alumina coatings using Rietveld refinement. Surf Coat Tech 2010; 204(20): 3248–3253. DOI: 10.1016/j.surfcoat.2010.03.026.

11.

Lima

Marple

. From APS to HVOF spraying of conventional and nanostructured titania feedstock powders: a study on the enhancement of the mechanical properties. Surf Coat Tech 2006; 200(11): 3428–3437. DOI: 10.1016/j.surfcoat.2004.10.137.

12.

Ramachandra

Lee

Tewari

. Durability of TBCs with a surface environmental barrier layer under thermal cycling in air and in molten salt. Surf Coat Tech 2003; 172(2-3): 150–157. DOI: 10.1016/S0257-8972(03)00260-3.

13.

Helminiak

Yanar

Pettit

, et al. Factors affecting the microstructural stability and durability of thermal barrier coatings fabricated by air plasma spraying. Mater Corr 2012; 63(10): 929–939. DOI: 10.1002/maco.201206646.

14.

Song

Zhang

Lin

, et al. Microstructures and residual strain/stresses of YSZ coatings prepared by plasma spraying. Mater Lett 2019; 240: 217–220. DOI: 10.1016/j.matlet.2018.12.126.

15.

Stecura

. Effects of compositional changes on the performance of a thermal barrier coating system.[for aircraft gas turbine engines]. In: Annual Conference on Composites and Advanced Materials, USA, 1 January, pp. 32.

16.

Woo

Lee

Jeong

, et al. Evaluation on cavitation characteristics of CoNiCrAlY/ZrO2–Y2O3 coating layer by atmospheric pressure plasma coating process. Mater Res Bullet 2014; 58: 78–82. DOI: 10.1016/j.materresbull.2014.03.026.

17.

Żórawski

The microstructure and tribological properties of liquid-fuel HVOF sprayed nanostructured WC–12Co coatings. Surf Coat Tech 2013; 220: 276–281. DOI: 10.1016/j.surfcoat.2012.11.007.

18.

Rovatti

Lecis

Dellasega

, et al. Influence of aging in the temperature range 250–350° C on the tribological performance of a WC-CoCr coating produced by HVOF. Int J Ref Met Hard Mater 2018; 75: 218–224. DOI: 10.1016/j.ijrmhm.2018.04.017.

19.

Zafar

Sharma

. Microstructure and wear performance of heat treated WC-12Co microwave clad. Vacuum 2016; 131: 213–222. DOI: 10.1016/j.vacuum.2016.06.021.

20.

Shan

Jiang

, et al. Effect of Ni addition on microstructure of matrix in casting tungsten carbide particle reinforced composite. J Mater Sci Tech 2013; 29(8): 720–724. DOI: 10.1016/j.jmst.2013.03.025.

21.

Zhou

, et al. High temperature wear performance of HVOF-sprayed Cr3C2-WC-NiCoCrMo and Cr3C2-NiCr hardmetal coatings. Appld Surf Sci 2017; 416: 33–44. DOI: 10.1016/j.apsusc.2017.04.132.

22.

Gan

Berndt

. Nanocomposite coatings: thermal spray processing, microstructure and performance. Int Mater Rev 2015; 60(4): 195–244. DOI: 10.1179/1743280414Y.0000000048.

23.

Subhash

Awasthi

Kunka

, et al. In search of amorphization-resistant boron carbide. Scrip Mater 2016; 123: 158–162. DOI: 10.1016/j.scriptamat.2016.06.012.

24.

Bor

Ichinkhorloo

Uyanga

, et al. Cu/CNT nanocomposite fabrication with different raw material properties using a planetary ball milling process. Powd Tech 2018; 323: 563–573. DOI: 10.1016/j.powtec.2016.06.042.

25.

Sarkar

Das

. Dry sliding wear characteristics of carbon nanotube/alumina nanocomposites under a sharp pyramidal indenter. Cera Int 2014; 40(9): 13971–13978. DOI: 10.1016/j.ceramint.2014.05.120.

26.

Balani

Bakshi

Chen

, et al. Role of powder treatment and carbon nanotube dispersion in the fracture toughening of plasma-sprayed aluminum oxide—carbon nanotube nanocomposite. J Nanosci Nanotech 2007; 7(10): 3553–3562. DOI: 10.1166/jnn.2007.851.

27.

Khor

. Crystallization behaviors in the plasma-spheroidized alumina/zircon mixtures. Mater Lett 2001; 48(2): 57–63. DOI: 10.1016/S0167-577X(00)00280-9.

28.

Basha

Srikanth

Venkateshwarlu

. Effect of reinforcement of carbon nanotubes on air plasma sprayed conventional Al2O3-3% TiO2 ceramic coatings. Mater Tdy: Proc 2020; 20: 191–194. DOI: 10.1016/j.matpr.2019.11.025.

29.

Jambagi

Kar

Brodard

, et al. Characteristics of plasma sprayed coatings produced from carbon nanotube doped ceramic powder feedstock. Mater Dsgn 2016; 112: 392–401. DOI: 10.1016/j.matdes.2016.09.095.

30.

Thakare

Mulik

Mahapatra

. Effect of carbon nanotubes and aluminum oxide on the properties of a plasma sprayed thermal barrier coating. Cera Int 2018; 44(1): 438–451. DOI: 10.1016/j.ceramint.2017.09.196.

31.

Yazdani

Tima

Mahboubi

. Investigation of wear behavior of as-plated and plasma-nitrided Ni-B-CNT electroless having different CNTs concentration. Appld Surf Sci 2018; 457: 942–955. DOI: 10.1016/j.apsusc.2018.07.020.