Material Microsurgery: Selective Synthesis of Materials via High-Temperature Chemistry for Microrecycling of Electronic Waste.

This study aims to establish a novel pathway for transforming complex electronic waste into advanced hybrid materials by leveraging high-temperature reactions. This research utilized silica (SiO2) sourced from computer monitor glass; carbon obtained from plastic components of spent monitor shells; and copper (Cu) recovered from waste printed circuit boards (PCBs) to produce a high-quality hybrid layer on a steel substrate. The transformation process consisted of two steps. In the first step, silicon carbide (SiC) nanowires were produced from the spent monitor's glass and plastic. In the second step, these nanowires were combined with Cu obtained by grinding waste PCBs to produce the hybrid layer over the steel surface. The Cu-SiC hybrid layer on a steel substrate was produced successfully by the judicious selection of waste sources and by selecting a microrecycling technique, which resulted in superior mechanical properties for the end product. This technique, proposed as 'material microsurgery', has the potential to transform waste materials into new hybrid surface coatings, which endows the base materials with superior properties to those seen in the source materials. For example, the SiC-nanowire-reinforced Cu layer added to steel in this study improved the hardness of the base material.

Introduction

Generally, used materials are recyled to produce similar products. For example, glass or plastic containers are recycled into more glass or plastic containers. However, complex waste streams such as electronic waste (e-waste) are composed of a variety of materials, which are difficult to transform into their original form. Glass, metals, and plastics are embedded in e-waste in such a manner that it is not feasible to separate them and remove contamination for conventional recycling.[1] Conventional recycling also requires a large input volume of like materials, which is not possible to obtain from many e-waste sources such as printed circuit boards (PCBs). Microrecycling takes a different approach to the conversion of waste materials through selective thermal transformation processes.[1] Through these processes, the metals, polymers, and ceramics contained in e-waste can be extracted and reused. Selective thermal transformation has the potential to support innovative processes to produce value-added products such as metal alloys and ceramics from e-waste. Microrecycling research distinguishes the selective thermal transformation processes of waste materials at a rudimentary level. This allows researchers to develop processes to selectively break and reform the bonds between various elements to form a new structure—creating “green”, value-added resources and products.[1] Through this approach, researchers identify many pathways through which the elements of waste materials react with each other and leverage these pathways to transform them into new products, which are completely different from the parent materials, and make them suitable for distinct applications. Using microrecycling to achieve targeted, small-scale recycling has the potential to sustainably reform hazardous e-waste and reduce environmental pollutants.[1−3]

In the selective thermal transformation technique, complex waste is charged in a closed-tube furnace and heat-treated in an oxygen-free atmosphere.[2−4] By tailoring the time and temperature, selective recovery of organics and inorganics is possible.[3,4] The selective synthesis of materials based on high-temperature thermal transformation will open novel pathways, not only to recovering valuable materials but also to generating new resources for industry.[5,6] This study transforms end-of-life materials that cannot be reused or recycled in a conventional manner, using a new microrecycling-based solution called “material microsurgery”. The material microsurgery technique is inspired by the principles used by medical surgeons when operating on the human body. Medical surgeons use operative protocols and instrumental techniques on patients to investigate and treat diseases or injuries—repairing the body or improving the way it functions. This study investigated end-of-life e-waste to identify valuable elements. Each component was uniquely treated to transform it into a value-added material. These materials can used to repair, restore, or enhance the properties of other materials to suit the needs of various applications. In this case, waste PCBs were reformed into a high-performance hybrid layer to modify a steel surface to achieve superior mechanical properties.

E-waste contains a number of valuable resources, which could be used to generate value-added materials for industrial applications. For example, various sources of e-waste contain a high proportion of glass. One such source is obsolate computer monitors. These monitors contain a significant amount of SiO2,[5] which is a rich source of Si to generate value-added nanoceramics via microrecycling techniques.[5,6] E-waste also contains many plastic parts (for instance, computer monitor shells), which are a good source of carbon.[5,7,8] Obsolete PCBs commonly contain around 40% metals (including approximately 36% Cu), 30% organics, and 30% ceramics.[9] The nonmetallic part of waste PCBs is rich in Si- and Al-enriched compounds, such as SiO2 and Al2O3.[9]

This research selected two sources of e-waste to generate a protective and high-performance layer on a steel surface. The first source was obsolete monitor glass and plastic shells, and the second was waste PCBs. Previous studies have shown that glass and plastic from e-waste can be transformed together through selective synthesis of material via the microrecycling technique of thermal isolation to generate high-value ceramic products such as SiC and Si3N4.[1,5] Cu can be recovered via a thermal route from waste PCBs. Cu has good conductivity, machinability, and corrosion resistance at ambient temperatures. These properties mean that Cu is used in a variety of applications, including electrical appliances,[3] the building industry, machinery, and the transport sector.[10] Cu in its pure form is not suitable for surface engineering because of its low tensile strength and poor wear resistance.[11] However, the superior corrosion resistance of Cu could be beneficial for surface engineering. It is established that the poor mechanical properties of Cu could be tailored and enhanced by reinforcing with nanoceramics, for example, SiC, SiO2, Si3N4, CeO2, Y2O3, CNT, and Al2O3 particles.[12−17] A ceramic-reinforced Cu surface coating created through selective synthesis from e-waste could increase the corrosion and wear resistance of the metal substrate. This method could be a suitable alternative to conventional surface coatings and reduce the burden on raw-material feedstocks.

There are previous examples of modifying metal surfaces with coatings sourced from waste to enhance the property of the parent material.[18−20] Previously, automotive waste was used to generate a hybrid layer on a high-carbon steel surface to obtain ultrahardness without compromising the toughness of the base material.[19,21] This surface layering improved steel’s performance by increasing wear and corrosion resistance under harsh operating conditions.[22] In this study, glass and plastic from spent computer monitors and Cu from waste PCBs were used to produce a hybrid thin film layer on a steel substrate without affecting substrate’s bulk properties. This material microsurgery approach provides a protective layer over the base material at the microscale to alter the overall performance of the material. It offers a sustainable technology that potentially opens up new applications of spent computers and replaces more expensive raw materials. The high-performance advanced material suggested here—synthesized using e-waste as the raw material—is a hybrid layer chemically bonded in situ on a steel surface to produce an innovative advanced material with superior mechanical properties to the uncoated steel.

Results and Discussion

Table shows the tentative percentages of metallic elements found in the waste PCBs before thermal transformation (using ICP–OES analysis). The analysis reveals that Cu is the dominant metallic element. X-ray fluorescence (XRF) analysis of the chemical composition of the waste PCBs and glass fraction of the waste computer monitors (Table ) indicates that there is a substantial amount of SiO2 present in both of these waste sources. Elements with less than 0.1 wt % are not included in the tables.

Table 1

ICP–OES Analysis of the Elemental Composition of the Metallic Components of PCBs

element	Cu	Sn	Zn	Fe	Ni	Pb	Ti	Al	total metal
wt %	24.6 ± 0.2	4.4 ± 0.2	1.3 ± 0.1	3.3 ± 0.1	0.4	0.4	0.2	2.0	36.6 ± 0.7

Table 2

XRF Analysis of the Chemical Composition of PCBs and Glass Fraction of Computer Monitors

oxides wt %	PCB plastic	oxides wt %	monitor glass
SiO2	36.8 ± 0.1	SiO2	68.3 ± 0.5
CuO	16.2 ± 0.1	Na2O	11.9 ± 0.1
CaO	12.7 ± 0.1	CaO	7.8 ± 0.1
Al2O3	11.4 ± 0.1	MgO	3.5
SnO2	4.3 ± 0.1	SO3	3.9
MgO	0.7	Al2O3	2.8
P2O5	0.2	Fe2O3	0.1

Figure a shows the spectrum obtained from Fourier transform infrared spectroscopy (FTIR) analysis of ground plastics from spent computer monitors. The peaks at 736, 750, and 830 cm–1 signify the CH out-of-plane deformation of meta- and ortho-disubstituted benzenes in polycarbonate. Three peaks at 1000, 1032, and 1182 cm–1 suggest the stretching mode of the C–O–C functional group. There are several peaks at 1233, 1453, 1507, 1620, and 1734 cm–1, ascribing the existence of the C=O/C=C functional group. The triplex at 2850, 2940, and 2980 cm–1 results from C–H plane bending. The peaks at 1734, 1233, and 830 cm–1 correspond to phenoxy resin, polyvinyl acetate, and vinyl chloride, respectively, while 1507 and 1453 cm–1 correspond to the phenoxy resin. The peak at 1182 cm–1 is because of phenoxy resin and vinyl chloride, and the peak at 1032 cm–1 is because of several polymers, especially vinyl acetate and vinyl chloride. The peak at 1009 cm–1 is from the phenoxy resin, and the other two peaks at 830 and 736 cm–1 also correspond to the phenoxy resin. Studying the analysis over the functional groups and their corresponding peaks, three types of polymer (phenoxy resin, polyvinyl acetate, and vinyl chloride) were evident in the plastic fraction of the waste computer monitors. A high-resolution XPS spectrum of the plastic shell of the computer monitor is represented in Figure b, where the C 1s region demonstrates two peaks at 284.8 and 286.4 eV, which also conform to C–C and C–O bonds, respectively.

Figure 1

(a) FTIR spectra and (b) high-resolution XPS spectra of the carbon region of the plastic shell of the computer monitor.

After heat treatment, the microstructure and phases of the SiC and the hybrid layer were characterized by their X-ray diffraction (XRD) patterns. The XRD pattern of the material produced after heat treatment with monitor shell plastics and ground glass revealed significant peaks of β-SiC. There were insignificant peaks corresponding to graphitic C and β-SiC whiskers, which usually result from stacking faults of the SiC nanowire microstructure.[23−25] In contrast, the hybrid layer on the surface of the steel substrate—derived from waste PCBs and the β-SiC from e-waste glass and plastics—showed clear peaks of Cu. The rest of the small peaks corresponded to C and stacking faults in the whisker structure of the β-SiC nanowires.

Figure a shows the XPS survey spectra of the Cu–SiC layer. As demonstrated, the photoelectron peaks at 100.98 eV and 102.97 eV correspond to Si 2p, as shown in Figure b, and 283.4, 285.78, 282.90, and 286.79 eV can be indexed to C 1s (Figure d). The peak of Si 2p is largely attributed to Si–C. A small amount of Si3N4 could be present in the coating. The presence of O with Si was also detected in small amounts. This was likely to be because of the SiC on the surface absorbing some oxygen molecules. The absorbed oxygen molecules react with the existing SiC. XPS revealed the chemical bonding between Si, C, O, and/or N—which is unsaturated and detected at several points on the surface layer after heat treatment. This phenomenon has been reported in other literature, where the surface coating of the metal has a combination of C, O, Si, and N.[26] The deconvoluted peaks of C could be assigned to the C–Si, C–C, and C–O bonds. Different kinds of nitrogen bonded with Si are present in the system, which is evident from the deconvolution of the high-resolution XPS spectra of N 1s (Figure g). The asymmetric N 1s spectrum was deconvoluted into two spectra: Si–N at 397.86 eV and pyridinic N at 398.9 eV.[27] The peak of Al 2p corresponds to Al2O3 (Figure f). The deconvoluted peaks of O at 531.7 eV can be attributed to Al2O3 and silicon oxynitride (Figure e). The photoelectron peaks at 932.66 eV, 934.31 eV, 939.96 eV, and 943.72 eV correspond to Cu 2p3, which could be attributed to Cu, CuO, copper, and CuO, respectively (Figure c). Table summarizes the possible compounds detected by XPS of the hybrid layer on the steel surface, related to the corresponding binding energy and photoelectron lines.

Figure 3

XPS spectra for the SiC–Cu hybrid layer and (a) survey scan of (b) Si 2p, (c) Cu 2p3, (d) C 1s, (e) O 1s, (f) A 2p, and (g) N 1s.

Table 3

Elemental Compositions on the Hybrid Layer Estimated by XPS

element	binding energy (eV)	photoelectron line	possible compounds/elements	references
Al	74.93	2p	Al2O3	(28)
C	283.4	1s	SiC	(29)
C	285.78	1s	C–C, SiC	(30)
C	282.90	1s	SiC	(31)
C	286.79	1s	C–O	(32)
N	398.9	1s	pyridinic N	(27)
N	397.86	1s	Si3N4	(33)
O	531.7	1s	Al2O3	(34)
O	533.7	1s	Si–O–C	(35,36)
Si	100.98	2p	SiC, Si3N4	(37)
Si	102.97	2p	Si–O–C, Si–N	(26)
Cu	932.66	2p3	copper	(38,39)
Cu	934.31	2p3	CuO	(40)
Cu	939.96	2p3	copper	(41)
Cu	943.72	2p3	CuO	(40)

A microstructural study of the SiC produced at 1500 °C was conducted by means of FE-SEM and TEM examination. The FE-SEM and TEM illustrations of the SiC, as shown in Figure a,b, show that the SiC appeared in uniform nanowire morphologies, and these nanowires have smooth surfaces with a diameter between 10 and 50 nm and are a few micrometers in length—nanowires with a similar structure and composition are also found in previous work.[23,24] In Figure c, lattice fringes with an interlayer distance of 0.255 nm correspond to the spacing between (111) planes of β-SiC (XRD Reference code: 03-065-0360). XRD patterns (Figure a) also show strong peaks in the (111) β-SiC phase. The SiC nanowires are clearly visible in the Cu matrix, as shown in Figure e,f. The red arrows in the bright-field TEM image show that the nanowires form a columnar structure in the Cu matrix during heat treatment, which forms a uniform layer over the steel surface. The corresponding dark-field image (Figure e) shows the contrast between the two materials (SiC and Cu) in the hybrid layer. Very dense SiC nanowires are uniformly embedded in the copper matrix, which will enhance the properties of the hybrid layer and create a protective layer on the metal substrate. The authors have only designated the SiC phases in the selective area diffraction (SAED) pattern of the layer. There are other rings present in the SAED pattern, which belong to the copper matrix.

Figure 4

(a) FE SEM image, (b) TEM image, and (c) HR TEM image of the SiC nanowires derived after first-step heat treatment. (d) TEM image of the hybrid surface layer with the SAED pattern derived after second-step heat treatment. (e) Bright-field TEM image of the layer showing SiC nanowires in the Cu matrix, and (f) corresponding dark-field image of Figure e.

Figure 2

XRD of (a) SiC nanowires derived from the glass and plastic e-waste and (b) Cu–SiC hybrid layer derived from e-waste. (β-SiC reference code: 03-065-0360, Cu reference code: 04-016-6874).

Figure shows the elemental distributions of Si, Al, Cu, C, and Fe obtained by TEM EDS elemental mapping of the hybrid layer produced by material microsurgery of e-waste—a novel approach explained in greater detail in the next section. A uniform concentration of Cu, Si, Al, and C is present within the hybrid layer, which appears to be less rich in Al. A very small peak of Al was also detected in the XPS spectra because of the presence of Al2O3 from powdered PCBs (Table ). However, XRD analysis could not confidently detect any Al-containing phase as the Al-containing phase must be less than 3% of the surface layer.

Figure 5

TEM EDS elemental mapping of the hybrid layer and substrate.

To evaluate contact-damage resistance of the hybrid layer, nanoindentation tests were carried out with a spherical indenter, where the peak load was chosen to cause deformation in the surface layer was up to 500 mN. A matrix of 3 × 3 indents was made on the layer for observation and analysis. The secondary electron microscopy (SEM) image of the indents and the focused ion beam (FIB)-milled cross-sectional image are shown in Figure a–c. The surface and cross-sectional morphologies of the spherical indents did not show any evidence of major cracks, holes, or flaking inside or outside of the hybrid layer. The interfacial bonding between the hybrid layer and the substrate was not affected by the indentation load. Although there was a major plastic deformation in the substrate indicated by the yellow arrow, as shown in Figure c, the structure of the hybrid layer was well maintained and did not discontinue throughout the damaged area (Figure c,d).

Figure 6

(a) Top view of ion-induced secondary electron images of an array of indents, (b) indent in the hybrid layer at a maximum load of 500 mN, (c) cross-sectional view of the ion-induced SE image of 500 mN indent, and (d) series of the TEM image indicating cross-sections of 500 mN. The green arrow indicates the SiC–Cu-based hybrid layer, and the yellow arrow shows the base steel.

Visible microcracking in the surface and subsurface underneath the indent was not evident. This was confirmed through microstructural analysis of the synthesized hybrid layer under indentation deformation. Severe plastic deformation for both the layer and substrate can be seen; however, the overall integrity of the hybrid-layering system is well maintained. No delamination between the hybrid layer and the substrate can be observed. This phenomenon reveals that the major plastic deformation created by normal loading through indentation is accommodated by the hybrid layer. Nonetheless, the resultant deformation energy of the hybrid layer is not dispelled in a brittle manner—as crack initiation and propagation were not observed.

Figure shows the load–displacement curves of the medium steel substrate, pure Cu, and the Cu–SiC hybrid layer under the same indentation load of 8 mN. The hardness and elastic modulus were calculated from the load–displacement curves and are demonstrated in Figure . The 95% confidence interval for the average value of the hardness and elastic modulus has been calculated from 100 data points of each sample, as illustrated in Figure b. When the material is hard, the indenter penetrates less, and the indentation depth is also lower compared to a softer material. This is clearly evident from the measurement of the maximum penetration depth, hmax. The minimum hmax was observed for the hybrid layer. Pure Cu is the softest material, as shown in Figure a—as it shows the maximum indentation penetration depth. However, when pure Cu was combined with SiC nanowires, the hardness and elastic modulus of the material increased.

Figure 7

(a) Nanoindentation load–displacement curve and (b) corresponding nanohardness and elastic modulus of the hybrid layer, steel substrate, and pure Cu.

Microrecycling Mechanism for Material Microsurgery

In this research, selectively mixed waste sources were combined to form a Cu–SiC-based hybrid layer. The waste sources and the process parameters were chosen in such a way that the hybrid layer could be produced as a coating on a steel surface to enhance its properties. This study was inspired by the principles used by medical surgeons when operating on the human body. Medical surgeons use operative protocols and instrumental techniques on patients to investigate and treat diseases or injuries—repairing the body or improving the way it functions. This study investigated end-of-life e-waste to identify valuable elements, in this case, Cu, C, and Si. These elements were uniquely treated to transform them into a value-added material, which could be used to repair, restore, or enhance the properties of another material. In this case, a Cu–SiC hybrid layer was formed on the steel substrate. The study was intended to demonstrate that, even though metals and ceramics have very different behaviour, the two materials can be bonded together through selective synthesis of material with the help of selective thermal transformation. The authors have named this technique “material microsurgery” because here we are working toward new surfaces on materials to create properties not evident in the parent materials. In this study, the hardness of steel was improved through coating with a nano-SiC reinforced Cu layer. The average nanohardness and effective elastic modulus of the steel surface without the modification treatment are 5.3 and 220 GPa, respectively. These values are significantly improved in the case of the modified surface: to 12.0 and 248 GPa (Figure b). The hardness of steel’s has increased by ∼125%, greatly enhancing its mechanical properties. This technology has been designed to be implemented in e-waste MICROfactories, which are small-scale, decentralized recycling centers that do not require a large input volume of waste materials. The term ‘micro’ has been used to describe the small-scale recycling technique employed.[42] In the material microsurgery technique described in this study, materials for the hybrid layer are extracted from problematic waste sources, which can only be reformed at a much smaller scale than that required for conventional recycling processes.

The formation of the Cu–SiC hybrid layer on the substrate is illustrated in Figure . The transformation process consisted of two stages. In the first stage, glass and plastic powder from the waste computer monitors were mixed for the first-step heat treatment—which was carried out in an inert atmosphere at 1500 °C—to obtain the SiC nanowires. The mechanisms of SiC formation from glass and plastic were investigated by several researchers and were found to be a two-stage reaction, nucleation and growth process.[5,6] During heat treatment of the e-waste, the plastic started to decompose, and carbon-saturated gas was generated. The C–C bond started to break down, and C reacted with SiO2 to form CO and CO2. In the second reaction, the gaseous silicon monoxide reacted with solid C and formed SiC nuclei. The gaseous silicon monoxide deposited over the nuclei continuously and subsequently reacted with the solid C to form SiC. When the atmosphere was saturated with CO, SiO gas reacted with CO and formed SiC nanowires.

Figure 8

Schematic of the formation of the SiC nanowire by thermal transformation and the formation of Cu-reinforced SiC hybrid layer on the steel substrate.

In the second stage of the heat treatment, SiC nanowire powder was mixed with ground PCBs. The steel substrate was covered with this mixture in the alumina crucible and heat treated at 1000 °C. The organic part of the PCB started to degrade at 300–400 °C and produced carbon-saturated gas (CH4, CO, and CO2).[3] At temperatures above 723 °C, iron is in an austenitic phase, which has the solubility of C of approximately 2%, where the steel substrate has only 0.32% C. In this phase, C from CH4 and CO starts diffusing into the steel surface through a gas–solid chemical reaction.

Carbon enrichment of the steel continues until the temperature reaches 1000 °C. When the furnace reached 1000 °C, the Cu melted and formed a Cu–SiC layer over the steel substrate. Failure at the interface of hard nanocomposite coating and soft substrate often occurs because of the mismatch of mechanical properties. In this case, this was avoided via a gradient structure, where a hybrid layer was formed in situ. First, C diffused to the steel substrate, and the hardness of the steel surface increased as the C content increased. At the surface of the steel, the hardness was expected to be greatest where the hybrid layer grew. This gradient structure minimizes the mismatch of strength between the hybrid layer and the metal substrate, which has the potential to prevent crack initiation and propagation through the coating and the substrate.

Conclusions

This study proposes a radical, new approach to the problem of managing the growing, global load of e-waste: material microsurgery. This research seeks to understand how many components of e-waste behave—at the molecular and nanolevels—and how the reactions triggered by controlled heat can be beneficially employed. These process parameters were used as an operative manual to coal a metal surface with a high-quality surface layer derived from two different sources of problematic e-waste. This was achieved by reforming glass and plastic from obsolete computer monitors into SiC nanowires via selective heat treatment. These nanowires and carbon residue were mixed with Cu from ground waste PCBs to produce a protective hybrid layer on the metal substrate. XRD, FE SEM, and TEM analyses were used to characterize the structure and morphology of the SiC-reinforced Cu material of the hybrid layer. The nanoindentation on the layer revealed good adhesion between the hybrid layer and the substrate. No cracking or chipping of the layer was visible in the microstructural analysis of the indentations. The hybrid layer increased the surface hardness by ∼125% compared to the base steel. Further development of material microsurgery techniques will provide solutions that allow the transformation of growing e-waste stockpiles into green, value-added surface coatings, which enhance the mechanical properties of the parent materials, making them suitable for a wider range of applications and operating environments.

Experimental Section

Metarial and Heat Treatment Methods

Multilayer waste PCBs and waste computer monitors were received from the Reverse E-waste company, in Sydney, Australia. After the waste was dismantled, the glass fraction from the computer monitors was pulverized using a ring mill. The plastic components of the computer monitors and PCBs were ground separately with a cryomill. The ground-glass and plastic fractions of the computer monitors were mixed and heat-treated in inert conditions at 1500 °C for 30 min in a horizontal tube furnace to produce SiC. The SiC and C residues produced by the mixed glass and plastics were combined with the ground PCBs to produce a SiC reinforced copper layer on an industrial-grade medium carbon steel substrate. The steel substrate was put in an enclosed crucible and covered with the crushed and ground e-waste in inert conditions within the furnace. Heat treatment was carried out in nitrogen gas-induced inert conditions. The flow rate of the gas was 1 L/min, and the temperature for the second step was 1000 °C for 15 min in a horizontal tube furnace. An alumina crucible was used in this research. The alumina crucible was covered but not fully sealed. After the final heat treatment, the sample had some carbon residue. XPS analysis of the residues is provided in the Supporting Information for this manuscript (Figure S1).

Characterization Methods

Prior to heat treatment, the chemical composition of the ground PCBs was identified via Perkin Elmer OPTIMA 7300 inductively coupled plasma—optical emission spectroscopy (ICP–OES). The metal samples were digested/extracted using mixed acid (HCL + HNO3), followed by open digestion for ICP–OES. For representative samples and statistical data reliability, each experiment was repeated five times. The glass fraction of the computer monitors was investigated with an X-ray fluorescence (XRF) analyzer. The sample was ashed at 815 °C for a minimum of 8 h before carrying out the XRF analysis. WD-XRF analysis was used in this case. The instrument used was an AXIOS Advanced WD spectrometer, manufactured by Malvern PanAlytical, which has a Rh tube (maximum power 4 kW). Optimum operating parameters for crystal, detector, angle, and power were used for each element. The program used for major elements (expressed as oxides) was called WROXI (wide ranging oxides). The calibration standards were prepared from pure synthetic oxides according to a precise formula. Ground plastics from the computer monitor were investigated using a FTIR and X-ray photoelectron spectrometer (XPS, Thermo ESCALAB250Xi). XPS spectra were obtained in situ with a monochromatic Al Kα X-ray source operating at 300 W and a hemispherical analyzer that collected data over a spot size of ∼1 mm to assess chemical bonding. This large spot size was selected to minimize the effects of small-scale inhomogeneities on composition measurement. Pass energies were 80 eV for the survey scans and 20 eV for the individual element, and adventitious C was removed by argon-sputtering. The synthesized SiC and the hybrid layer were characterized via PANalytical Empyrean X-ray Diffraction with Cu-Kα radiation (λ = 1.54060 Å) at 45 kV and 40 mA. Observed XRD ranged from 20 to 110°. For the layered structure, the selected incidence angle was 3°. This low-grazing incident angle helped to overcome interference from the steel base material. The elemental concentration and the microstructural analysis of the structure were detected with a transmission electron microscope (Philips CM 200, FEI Company, Oregon, USA) and incorporated with energy-dispersive X-rays. The cross-sectional structure of the samples was also observed with a focused ion beam (FEI xT Nova Nanolab 200, FEI Company, Hillsboro, OR, USA) microscope coupled with SEM. Nano SEM, a field-emission scanning electron microscope (FEI Nova NanoSEM 450, FEI Company, Hillsboro, OR, USA), was used to investigate the nanostructure of the synthesized SiC. TEM sample preparation was carried out using FIB milling. To determine the nanohardness and elastic modulus, the authors analyzed five sets of 5 × 5 matrix at a load of 0–8 mN and included results in the revised manuscript. Matrices of 3 × 3 indents were used to demonstrate contact damage resistance of the hybrid layer. The interfacial bonding between the ceramic layer and the substrate was not affected by the indentation load. Although there was a major plastic deformation in the substrate, the structure of the hybrid layer was well maintained and did not discontinue throughout the damaged area.

A UMIS (Ultra-Micro Indentation System 2000, CSIRO, Sydney, Australia) workstation equipped with a Berkovich diamond indenter was used to measure the mechanical properties of the hybrid layer. The hardness (H) and elastic modulus (Er) of the coatings were calculated according to the Oliver–Pherr method.[43]