Fracture behaviour of ceramic–metallic glass gradient transition coating

Author’s Accepted Manuscript Fracture behaviour of ceramic–metallic glass gradient transition coating Qiaolei Li, Peng Song, Kaiyue Lü, Quan Dong, Qing Li, Jun Tan, Qingwei Li, Jiansheng Lu www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(18)33379-0 https://doi.org/10.1016/j.ceramint.2018.12.015 CERI20238

To appear in: Ceramics International Received date: 20 October 2018 Revised date: 26 November 2018 Accepted date: 3 December 2018 Cite this article as: Qiaolei Li, Peng Song, Kaiyue Lü, Quan Dong, Qing Li, Jun Tan, Qingwei Li and Jiansheng Lu, Fracture behaviour of ceramic–metallic glass gradient transition coating, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.12.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Fracture behaviour of ceramic–metallic glass gradient transition coating Qiaolei Li1, Peng Song1, Kaiyue Lü1, Quan Dong1, Qing Li1, Jun Tan1, Qingwei Li2, Jiansheng Lu1

1 Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China

2 Advanced Research Institute for Multidisciplinary Science, Qilu University of Technology，Jinan 250353, China

Abstract A new composite coating material of ceramic (Al2O3-40 wt.% TiO2)–metallic glass (Fe56Cr23Mo13B8) gradient transition coating was successfully prepared by atmospheric plasma spraying technology, and a new theory of laminar–columnar structure gradient transition synergistic enhancement effect was proposed. The microstructure and element distribution of the composite coating were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron probe microanalysis (EPMA). The fracture toughness and fracture behaviour of the composite coating were analysed via micro-hardness and three-point bending (3PB) tests. The results showed that the stress release of the ceramic–metallic glass gradient transition coating was stable compared with that of the conventional gradient coating in the stage of acute deformation, and the coating exhibited better fracture toughness. Different areas of the ceramic–metallic glass gradient transition coating exhibited 

Corresponding author: Tel.: +86 18288698196.

E-mail address: [email protected]. [email protected]. 1

different fracture behaviours. Additionally, the ceramic layer was made of columnar crystals, and the metal–glass layer was lamellar. The laminar–columnar structure gradient transition synergistic enhancement effect improved the anti-crack growth and fracture toughness. This study provides a new and viable option for the improvement of thermal spraying ceramic composite coatings. Keywords Ceramic–metallic glass coatings; Gradient transition; Laminar–columnar structure; Fracture toughness; Thermal spraying

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1. Introduction Al2O3 base ceramic coatings are widely used due to its high hardness and good wear-resistant [1–3]. However, Al2O3 coatings are brittle; thus, TiO2 is added to Al2O3 to increase the toughness and wear resistance [4–6]. Al2O3/TiO2 composite ceramic coatings possess many advantages compared with Al2O3 coatings owing to their excellent properties such as good toughness and better wear resistance [7–12]. In addition, thermal spraying is a common technique for preparing ceramic coatings owing to its flexible, fast, and cost-effective deposition [13–16]. Plasma-sprayed Al2O3/TiO2 ceramic coatings are generally applied to improve the wear, thermal insulation, and corrosion of metallic and alloy substrates [17–22]. Therefore, the application of ceramic coatings in aerospace machinery and other fields is extensive [8,10,20–22]. In many industrial applications, the material service is accompanied by temperature rise[23] and stress generation [24, 25]. Due to mechanical [18] and thermal mismatch [26, 27] between metallic substrates and ceramic coatings, weak zone between ceramics coating and substrates is typical. The adhesion of ceramics coatings with metallic substrates determines the service life and mechanical properties of ceramic-metal composite materials [26, 28, 29]. In order to enhance the adhesion between ceramic coating and substrate and the mechanical properties of composite coating materials, a large number of studies have been performed. The adherent performance between ceramic coating and substrate is improved by the preparation high density ceramic coating [30, 31, 32] and reduction of defects on the interface 3

between ceramic coating and bondcoat [33-35]. Further, heat-induced coupling layer between the metallic bondcoat and ceramic coatings [36], also improves the adhesion between them. In addition, the in-situ oxidation between the alloyed bondcoat and ceramics coatings [37] attenuates the interfacial lattice mismatch and improves the bonding strength of the ceramics and bondcoat. More than this, the metal-ceramic transition zones join the metallic bondcoat and ceramic coatings[2, 38], which reduces the stress gradient between the metal and ceramic, and enhances the adhesion between them. Although the aforementioned methods improve the adhesion between the metal and ceramic but the influence of the mechanical mismatch on the strength of ceramic coating is not completely reduced. Therefore, a materials possessing high strength and good wettability is essential to improve the mechanical properties of the ceramic composite coatings. It is well known that metal-glass coating are of scientific interest in material science field [39, 40] due to their unique mechanical, physical and chemical properties associated with the long-range disorder and short-range order atomic structure [41]. In this study, owing to metal-glass have high strength and good metal wettability, a ceramic (Al2O3-40 wt.% TiO2)–metallic glass gradient transition coating is prepared via a well-designed experiment, and the crack growth and fracture behaviour under the stress are studied. A theory of the laminar–columnar structure gradient transition synergistic enhancement effect is proposed. The fracture toughness and anti-crack growth property of the ceramic composite coatings are improved, and a new idea for the combined design of other metal–glass and ceramic materials is presented. 4

2. Experimental procedure 2.1 Experimental design In this study, a metallic glass coating with high strength is used as the bondcoat and an Al2O3-40 wt.% TiO2 (AT40) ceramic coating with excellent wear resistance is used as the top coating to improves the mechanical properties of ceramic composite coating material. The problem of adhesion between the ceramic and metallic glass coatings is solved by gradient transition. Considering the problem of bonding between the metal– glass coating and the substrate, Fe56Cr23Mo13B8 is chosen as the metal–glass bondcoat, whose composition is similar to that of the substrate. The thermal expansion coefficient (CET) of carbon steel is (1.543–2.071) × 10−5/°C [42], the CET of AT40 ceramics is −2.7 × 10−6 /°C [43, 44], and the CET of metal–glass materials is 8.5 × 10−6 /°C [45]. The difference in the CET between the medium carbon steel substrate and the AT40 ceramic coating is reduced by the metal–glass bondcoat. 2.2 Sample preparation The top ceramic coatings contained a commercially available Al2O3-40 wt.% TiO2 powder (Oerlikon Metco, Amdry 6244), and the powder size was approximately 20– 30 μm, which is the same as that used the same size used in a previous study [2]. The bondcoat was prepared using a commercially available metal–glass powder (Liaoning Jinxin Technology Co., Ltd.), with a powder size in the range of 10–50 μm. The medium carbon steel substrates were designed to be rectangular, with dimensions of 12 mm × 10 mm × 2 mm and 40 mm × 12 mm ×  2 mm. Before atmospheric plasma spraying, the substrate is cleaned using alcohol and sanded to obtain a clean and 5

rough surface for spray coating. The ceramic–metal glass gradient transition coatings are prepared via plasma spraying in two steps. First, the metal–glass powder is fed in a single path, forming a dense metal–glass coating in direct contact with the substrate. This compact metal–glass coating improves the adhesion of the coating to the substrate. In the second step, the gradient transition coating was prepared with the metal-glass powder and Al2O3-40 wt.% TiO2 powder via two symmetrical feed channels working together. Table 1 shows the spraying parameters. To prevent the metal–glass coating from crystallising during the spraying process, the samples were cooled to room temperature after the first spraying step, and the surface of the samples is cooled using compressed air throughout the spraying process. 2.3 Mechanical test 2.3.1. Micro-hardness test A micro-indentation test is performed after polishing the cross sections of the coatings to create and propagate cracks. In this test, a load of 200 g was utilized for 10 s. Then, the ten indentations are performed randomly, and the average coating hardness is calculated. The indentations are characterised by SEM. The fracture toughness was measured using the micro-indentation technique, which is based on the crack-propagation resistance of coatings. The relationship between the fracture toughness (Kf) and crack length (L) [46] is as follows: ⁄ ⁄

,

(1)

Where A is a constant that depends on the geometry of the indenter (A = 0.016 for micro-indentation test), E is the Young’s modulus, HV is the micro-hardness, and F is 6

the load during indentation. The fracture toughness of the coatings during the test is acquired using Equation (1). 2.3.2. Three-point bending (3PB) test In order to simplify the experiment, the stress of the coating is applied directly by 3PB test to study the crack growth and fracture behavior of the coating. A 3PB test (Instron 5848 Micro-Force Tester, Canton, MA) is performed to analyse the fracture behaviour of the ceramic–metallic glass gradient transition coatings. As shown in Fig. 1, the samples are designed to be rectangular, with dimensions of 40 mm × 12 mm × 2 mm. The span between the two supporting pins is 25 mm, and the middle of the sample is pressed by the load cell. In the testing process, the load cell moves down at a speed of 0.1 mm/min. Each sample is loaded on the load cell, until reaching a displacement of 1 mm. 2.4 Microstructural characterisation The samples are abraded and polished on their cross sections before performance testing and characterisation. The microstructure and element distribution of the sample cross sections are characterised using SEM (FEI Quanta 600) and EPMA (EPMA-1720, Shimadzu). The surfaces of the metal–glass coatings and ceramic– metallic glass gradient transition coatings is analysed using XRD (D8 Advance, Bruker, Germany). 3. Results and discussion 3.1 Microstructure analysis Fig. 2 shows SEM images for the cross-sectional of the ceramic–metallic glass 7

gradient transition coatings. The sample coatings consisted of the metal–glass coatings, the ceramic top coatings, and the metallic glass–ceramic transition zone between the metal–glass bondcoat and the ceramic top coatings, as shown in Fig. 2a. In the ceramic–metallic glass gradient transition zone, the metal–glass phase and ceramic phase had alternating distributions, the porosity was low, and no obvious defects were present, as shown in Fig. 2b. This glass gradient transition zone made the interface between the top ceramic coatings and the metal–glass bondcoat disappear. The interface between the gradient transition zone and the metal–glass coating was tightly bonded, and the content of the metal–glass phase from the metal–glass coatings to the ceramic coatings gradually decreased in the transition zone, as shown in Fig. 2c. The lamellar distribution characteristics of the metal–glass coatings were observed, and no defects were detected inside the lamella (Fig. 2d). The compact laminated metal–glass coatings prevented the entry of corrosive media and improved the corrosion resistance of the coating. Fig. 3 shows SEM images of the surface of the ceramic–metallic glass gradient transition coatings, which indicating that numerous micro-cracks on the top ceramic coatings were formed during spraying. When a coating is subjected to stress, this cracks will be the source of the coating fracture [47, 48]. 3.2 Phase analysis and element distribution Fig. 4 shows XRD data for the surfaces of the metal–glass coatings and the ceramic– metallic glass gradient transition coatings. The diffraction analysis revealed that only Al2O3, TiO2, and Al2TiO5 ceramic phases existed on the surface of the gradient 8

transition coatings. XRD analysis for the surface of the metal–glass coatings showed that most of the amorphous coating remained in the amorphous state, except for a small amount of crystallised Fe formed during the plasma spraying. Fig. 5 shows the EPMA analysis results for the ceramic–metallic glass gradient transition coatings. According to the analysis results shown in Fig. 4b, c, and d–f, the dark areas in Fig. 5a are mainly the Al-rich and Ti-rich ceramic areas, and the white areas are mainly the Fe-rich, Cr-rich, and Mo-rich metal-glass areas. A small amount of elemrnts B is present in the white area, which is consistent with the composition of metal–glass powder, as shown in Fig. 5g. The element O is distributed in the black region and is not observed in the white region, as shown in Fig. 5h. Combining these results with the XRD analysis results shown in Fig. 4, which reveals that the black area is the ceramic phase of Al2O3, TiO2, and Al2TiO5, and the white area is the metal– glass phase. From the perspective of the element distribution, the interlocking transition gradient zone of the ceramic phase and the metal–glass phase is formed between the ceramic top coating and the metal–glass coating, which makes the interface between the ceramic top coating and the metal–glass layer disappear. In the gradient transition zone, the phase of the ceramics gradually decreased, and the metal–glass phase gradually increased from the ceramic top coating to metal–glass coating. This structure can reduce the stress gradient of the ceramic coating and the metal–glass coating, which improving the adhesion of the coating during service. The microstructure and element distribution of the gradient transition zone are analysed in Fig. 6, on the basis of Fig. 5. The elements Fe, Cr, Mo, and B were evenly 9

mixed, and the phenomenon of single-element enrichment was unobserved, as shown in Fig. 6d–g. The distribution of elements O is completely consistent with that for ceramics, which indicating that the elements in the metal–glass phase underwent no oxidation during the spraying process, as shown in Fig. 6b, c, and h. In the transition zone, the metal–glass phase and the ceramic phase are alternated and in the compact lamellar structure, the metal–glass phase exhibits a laminar and granular distribution, and the ceramic phase is mainly laminated. The interface between the ceramic phase and the metal–glass phase is tightly bonded, without obvious defects. 3.3 Bending test Fig. 7 shows SEM images of the samples after the 3PB test. Fig. 7a shows macroscopic images of the sample after the 3PB test. A penetrating crack is observed on the surface of the samples. In the process of crack propagation, the penetrating crack has obvious deflection and bifurcation, as shown in Fig. 7b. This phenomenon differs from results of previous studies on metal–ceramic gradient coatings [2, 38]. In these studies, the cracks were straightedge in the process of passing through the entire surface of the samples. Fig. 7c and e shows cross-sectional SEM images of the coatings after the 3PB test. Here, the ceramic coating, metal–glass coating, and gradient transition zone can be clearly distinguished. The crack paths in the ceramic coating, gradient transition zone, and metal–glass coating are straight, zigzag, and stepwise, respectively. Fig. 7e–h shows that the fracture in the ceramic coating is along the crystal fracture; the fracture in the transition zone is tensile failure, with the pulling out and bridging of the metal–glass layer; and the fracture in the metal–glass 10

coating is brittle fracture. Fig. 8 shows the typical load–displacement curves of the ceramic–metallic glass gradient transition coatings. To illustrate the effect of the gradient transition coatings in the 3PB test, the substrate was also tested. The fracture process of the gradient transition coatings was divided into three stages according to the comparison with the substrate load–displacement curve. In the first stage, the load and displacement are linear. At this time, the substrate underwent elastic deformation, and the stress was not large enough to damage the coating. The displacement continued to increase and entered the second stage, where the ceramic coating began its slow crack propagation at the source of the surface micro-crack shown in Fig. 3, and the substrate remained in the elastic deformation stage. The micro-crack propagation led to the release of stress, which making the load–displacement curve of the ceramic–metallic glass gradient transition coatings below curve of the substrate. With the further increase of the displacement, the substrate entered the yield stage, and the cracks in the coating began to propagate; thus, the accumulated stress of the coating in the second stage was released smoothly. Finally, the coating was completely fractured, causing the load– displacement curve to intersect with the substrate curve again. Subsequently, the coating failed. The slope of the curve at the elastic stage is the elastic modulus of the material. 𝐸𝑐𝑜𝑎𝑡𝑖𝑛𝑔 𝐸𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒

𝑡𝑔𝛼

(2)

𝑡𝑔𝛽

(3)

α>β

(4) 11

Thus, 𝐸𝑐𝑜𝑎𝑡𝑖𝑛𝑔 > 𝐸𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒

(5)

Where, 𝐸𝑐𝑜𝑎𝑡𝑖𝑛𝑔 is the elastic modulus of the substrate with ceramic–metallic glass gradient transition coatings, 𝐸𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 is the elastic modulus of the substrate, 𝛼 is the angle between the tangent of the ceramic–metallic glass gradient transition coatings load–displacement curve and the horizontal in the elastic deformation stage, and 𝛽 is the angle between the tangent of the substrate load–displacement curve and the horizontal in the elastic deformation stage. Equation (5) expounds that the substrate with coatings has a higher elastic modulus than the substrate without coatings. The peak value for the sample with coatings in the second stage was 840 N, while that for the substrate was only 790 N. This is because of the resistance of the coating to the stress produced by the increased displacement, which increased the load of the sample, producing plastic deformation. The results show that the coating increased the strength of the material. In addition, as shown in the lower-right corner of Fig. 8, the stress release in the coating was more stable than that in the substrate, indicating that the gradient transition coating had greater toughness. Fig. 9 shows the load–displacement curves of the ceramic (AT40)–metallic glass and ceramic (AT40)–NiCoCrAl gradient transition coatings [38]. In the stress-release stage, the ceramic–NiCoCrAl gradient transition coating exhibited two rapid stress releases. First, stress was released by the rapid fracture of the top ceramic coating. Second, stress was released by the fracture of the NiCoCrAl coating. The phenomenon of the stress release step appeared, indicating that the stress difference between the alloy coating and the ceramic coating was not solved well through the 12

gradient transition coating; this requires further optimisation. The stress release of the ceramic–metallic glass gradient transition coatings was stable, indicating that the cracks in the ceramic coating, gradient transition zone, and metal-glass coating spread at a uniform rate. In the load–displacement curve of Fig. 8, the stress-release amount can be determined using the following equations. △ 𝐹𝑚𝑒𝑡𝑎𝑙−𝑔𝑙𝑎𝑠𝑠𝑙 ≈ 0

(6)

△ 𝐹𝑁𝑖𝐶𝑜𝐶𝑟𝐴𝑙

(7)

△ 𝐹1𝑁𝑖𝐶𝑜𝐶𝑟𝐴𝑙 +△ 𝐹2𝑁𝑖𝐶𝑜𝐶𝑟𝐴𝑙

△ 𝐹𝑚𝑒𝑡𝑎𝑙−𝑔𝑙𝑎𝑠𝑠𝑙 <△ 𝐹𝑁𝑖𝐶𝑜𝐶𝑟𝐴𝑙

(8)

Where, △ 𝐹𝑚𝑒𝑡𝑎𝑙−𝑔𝑙𝑎𝑠𝑠𝑙 is the total amount of stress release of the ceramic–metallic glass gradient transition coatings at the stress-release stage, △ 𝐹𝑁𝑖𝐶𝑜𝐶𝑟𝐴𝑙 is the total amount of stress release of the ceramic–NiCoCrAl gradient transition coatings at the stress-release stage, △ 𝐹1𝑁𝑖𝐶𝑜𝐶𝑟𝐴𝑙 is the amount of stress release of the ceramic– NiCoCrAl gradient transition coatings at the first stress-release stage, and △ 𝐹2𝑁𝑖𝐶𝑜𝐶𝑟𝐴𝑙 is the amount of stress release of the ceramic–NiCoCrAl gradient transition coatings at the second stress-release stage. Equation (8) indicates that the ceramic–metallic glass gradient transition coatings than the ceramic–NiCoCrAl gradient transition coating had a lower stress gradient and greater toughness. 4. Discussion 4.1 Fracture toughness Fig. 10 shows a typical SEM image of the indentations after the micro-hardness test. In the ceramic coating and metal–glass coating, 10 indentations were performed randomly, and the average micro-hardness values were 2,463.8 HV0.2 and 3,663.6 13

HV0.2, respectively. Near the indentation of the ceramic coating, a large number of tip and ring cracks and crushed splats were observed after the micro-hardness test, as shown in Fig. 10a. The tip cracks caused the stress-release, and the ring cracks caused crushed splats. Therefore, only the indentations of tip cracks were used for estimating the fracture toughness. The relationship between the fracture toughness and the crack length is given by Equation (1). To meet the geometric requirements of Equation (1) (the length of the crack after indentation should be at least twice the radius of the indentation), only indentation satisfying this requirement was used to estimate the fracture toughness. Measured the length of each crack in Fig. 10 to satisfy the aforementioned requirements, and calculated the average fracture toughness using Equation (1) and calculate the average value. The fracture toughness of the ceramic coating prepared in this study was calculated as 2.8 ± 0.22 MPa∙m1/2, which is reasonable. In previous studies, the fracture toughness of pure Al2O3 was 1.8 MPa∙m1/2, and that of Al2O3-50 wt.% TiO2 was 5.1 MPa∙m1/2 [48]. The fracture toughness obtained in this study is slightly lower than that previously reported, and the difference may be caused by the differences in the plasma-spraying parameters and substrate materials. After the micro-hardness test, cracks were produced in the metal–glass coating along the indentation tip, but the overall damage degree was less than that of the ceramic coating, as shown in Fig. 10b. Each crack that satisfied the calculation requirements of Equation (1) was selected for measurement. The fracture toughness was calculated using Equation (1), and the average value was calculated. The fracture-toughness value of the metal–glass coating prepared in this study was 3.6 14

± 0.32 MPa∙m1/2, which is consistent with the value of 3.2–3.8 MPa∙m1/2 previously reported for metal–glass materials (Fe48Cr15Mo14Er2C15B6) [49]. The results indicate that the metal–glass coating had higher crack-propagation resistance. The comparison with other materials is shown in the Ashby map of comparison diagram of the fracture toughness-yield strength [50, 51]. Compared with ceramic materials, metal-glass materials generally have higher fracture toughness. Furthermore, both ceramic and metal-glass materials have higher Young’s modulus. The analysis indicates that the metal-glass material as a bondcoat can greatly improve the resistance of crack-growth and strength of the ceramic composite coating. In addition, the bonding between the metal-glass coating and ceramic coating is enhanced by the gradient transition zone, and the stress gradient of the coating is reduced. 4.2 Fracture behaviour 4.2.1 Surface fracture behaviour Fig. 11 shows the crack evolution process under stress on the coating surface. The ceramic–metallic glass gradient transition coating formed a large number of micro-cracks on the surface of the coating (Fig. 11a and b), owing to the rapid cooling during spraying. In the second stage of the 3PB test, the crack started to expand along the boundary of columnar crystals in the ceramic coating with the micro-crack in Fig. 11b as the source, as shown in Fig. 11c and d. Columnar crystals were clearly observed, with diameters ranging from 75–150 nm, as shown in Fig. 11d. When the crack was exposed to the unmelted particles, it branched and bypassed the unmelted particles, as shown in Fig. 11e. The unmelted particles and a hard phase contributed to 15

the enhancement of the crack resistance of the coating. As the stress continued to increase, the intra-particle cracks between the flat particles in the coating began to spread inside the coating, as shown in Fig. 11f. In the gradient transition region, the crack encountered metal–glass particles with greater fracture toughness, causing it to bifurcate and bypass, as shown in Fig. 11g. Fig. 11 can be concluded that the micro-crack of the ceramic–metallic glass gradient transition coating surface was the source of the macroscopic fracture crack, and the crack extended along the columnar crystal in the ceramic coatings. The unmelted particles in the ceramic coating and the metal–glass particles in the gradient transition zone made the cracks bypass or bifurcate, improving the crack-propagation resistance of the coating. 4.2.2 Ceramic coating fracture behaviour

Fig. 12 shows an SEM image of the fracture surface for the ceramic coatings (Fig. 7f). The melted and semi-melted AT40 powder hit the substrate and formed a laminated structure (Figs. 12a and b). Due to the temperature difference between the substrate and thermal source during spraying, a distinct columnar crystal structure was formed inside the lamella (Fig. 12c). Previous reports [52] showed that the plasma-sprayed ceramic coating has the form of columnar crystals. The presence of interlamellar pores is usually due to thermal stresses in the process of thermal spraying deposition. The adjacent layers of the ceramic coating were partially separated by interlamellar pores, and the limited bonding area became the main carrier of transmission force [53]. To investigate the failure of the ceramic layer for the ceramic–metallic glass gradient transition coating, the propagation process of the crack was divided into two sections. 16

The strain produced by the 3PB test caused the entire coating to be stretched. Owing to the tensile stress in the lamella, the columnar grain boundary became the weak area in the flat layer. When the tensile stress was greater than the bonding strength of the grain boundary, the crack more easily propagated along the grain boundary. Soon afterwards, the first layer transferred strain to the top layer of the second layer, as shown in Fig. 12c–e. Owing to the presence of interlamellar pores between the first layer and the second layer, the interface became a more vulnerable area, and the cracks spread more easily along this interface. During strain transfer, the bonding area of the interface was stressed in shear form, resulting in the extension of the tensile gap. Fig. 12f shows the fracture behaviour model of the ceramic region. The tensile stress in the ceramic coating layer caused the crack to propagation along the columnar grain boundary. When the strain reached the second top surface, owing to the presence of interlamellar pores, the bonding area of the interface was subject to shear stress, and the crack started to spread along the top interface. 4.2.3 Gradient transition zone fracture behaviour Fig. 13 shows an SEM image of the gradient transition zone fracture surface (Fig. 7g). As shown in Fig. 13a, the metal–glass layers in the transition zone were distributed as thin splat, thick splat, and spherical. Fig. 13b shows the fracture behaviour of the metal–glass thin splat in the gradient transition zone. The small particles of the metal– glass material were hit by ceramic particles during deposition and existed in the coating in the form of a thin splat. Before the crack encountered the metal–glass thin splat, it spread along the boundary of the ceramic columnar grains. When the crack 17

encountered the metal–glass thin splat, causing splat deformed and broke under the large tensile stress, and this consumed energy during the crack growth. When a spherical particle of the metal–glass material was encountered during the crack propagation, the crack was deflected and bypassed it (Fig. 13c). Because the thick splat of metal–glass had high strength, it was not damaged by the crack propagation. It played a bridging role in the process of small deformation and was pulled out when the macro-crack was generated (Fig. 13d). The model of crack growth in the transition zone is shown in Fig.13e and f. The metal–glass thin splat was pulled apart by the crack, which consumed part of the energy of the crack growth. The metal–glass spherical particle made the crack turn and bypass, and the metal–glass thick splat was pulled out by the crack and bridged the gap. The metal–glass splats of different shapes in the transition zone contributed to the barrier crack growth of the coating. 4.2.4 Metal–glass coating fracture behaviour SEM images are the fractured surface of the metal–glass coatings in the Fig. 7h, as shown in Fig. 14. The metal–glass coatings present a typical layer structure, as shown in Fig. 2d. Importantly, the structure within the metal–glass layer differed from the columnar crystal structure of the ceramics, owing to structural characteristics of short-range order and long-range disorder. The metal–glass coating exhibited a stepped fracture (Figs. 14a and b). The fracture of the metal–glass occurred in not only one layer but also several layers, and multilayer deformation may be the cause of greater plasticity of metal–glass coatings, as shown in Fig. 14c. The results expound that the bond between the layers of metal–glass was reliable. The fracture of the 18

metal–glass lamella was flat, which expressed as brittle fracture (Figs. 7h and 14c). The fracture of the metal–glass coating was accompanied by the propagation of interlaminar cracks (Fig. 14d). The metal–glass coatings exhibited two types of fracture behaviour: stepped brittle fracture and interlayer cracks propagation. The metal–glass coating fracture exhibited a step shape, indicating brittle fracture of the layer and crack propagation of the interlaminar. This is because the metal–glass had a high Young’s modulus. According to the load–displacement curve obtained in the 3PB test, the load increased after the end of the stress release platform in the third stage. It can be judged that the secondary rise of the load was due to the strain transferred to the metal–glass coating, and the crack began to expand in the metal–glass coating. These characteristics of metal–glass give ceramic composite coating strong tensile properties and contribute to the improvement of the coating strength. 4.2.5 Laminar–columnar structure gradient transition effect In this study, an AT40 ceramic–metallic glass gradient transition coating was successfully prepared. This coating exhibited excellent resistance to crack growth and fracture toughness. A model diagram of the effect of the laminar–columnar structure in the ceramic–metallic glass gradient transition coating, as shown in Fig. 15. The ceramic coating has a columnar crystal structure, the metal–glass coating has a lamellar structure, and the gradient transition region has an alternating columnar-layered transition structure. The ceramic–metallic glass gradient transition coating has the synergistic enhancement effect of the laminar–columnar structure gradient transition. The columnar ceramic coating has excellent wear resistance, but 19

compressive stress resistance and tensile strength is poor. The metal–glass sheet has the strong tensile stress resistance owing to high strength and fracture toughness [50, 51]. The transition zone of the columnar-layered glass gradient transition zone perfectly bonds the two structures together, which improves the compressive and tensile properties of the overall coating. The composite coating consists of a ceramic top coating and a metal–glass bondcoat. The structure retains the wear-resistant properties of the ceramic top coating. The ceramic coating protects the metal–glass bondcoat, which alleviating the problem of the low stability of the metal–glass materials. This study demonstrated that the ceramic–metallic glass coating is implementable and has excellent anti-crack growth resistance. Compared with other materials, ceramic and metal–glass materials have excellent mechanical properties [50, 51]. In this study, although the ceramic and metal–glass materials used were the AT40 and Fe56Cr23Mo13B8, respectively, the results provide a new idea for combining other metal–glass and ceramic materials. 5. Conclusions Ceramic–metallic glass gradient transition coatings were successfully prepared using atmospheric plasma spraying technology. The following important conclusions were drawn. 1. In ceramic–metallic glass gradient transition coatings, the laminar–columnar structure gradient transition synergistic enhancement effect improved the fracture toughness and anti-crack growth resistance. 20

2. The gradient transition coating exhibited different fracture behaviours. In the ceramic layer, the cracks expanded along the columnar crystal boundary of the ceramic. In the transition zone, the fracture behaviours of the metal–glass splat include tensile failure, crack bypass, and the pulling out and bridging. In the metal– glass layer, the fracture mode was brittle fracture of the layer and interlaminar crack extension. 3. Metal–glass as a bondcoat was an effective material to improve the fracture toughness of the ceramic composite coating, which may be set as a viable new option for the improvement of the mechanical properties of ceramic composite coatings.

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Acknowledgements The authors acknowledge funding from the Yunnan Province Key Research and Development Program (Grant No. 2018BA067), Yunnan Province Science and Technology Major Project (Grant No. 2018ZE009), and National Natural Science Foundation of China (Grant No. 51401097).{Citation}

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Figure Captions

Fig. 1 Schematic diagram of the three-point bending (3PB) test. Fig. 2 Cross-sectional SEM images of the ceramic–metallic glass gradient transition coatings: (a) gradient transition coatings; (b) transition zone; (c) transition zonemetal–glass coating interface; (d) metal–glass coatings. Fig. 3 SEM image of the ceramic–metallic glass gradient transition coating surface. Fig. 4 XRD analysis of the coating surface. Fig. 5 EPMA mapping for the ceramic–metallic glass gradient transition coatings. Fig. 6 EPMA mapping for the transition zone. Fig. 7 SEM images of samples after the 3PB test: (a) samples after the 3PB test; (b) sample surface; (c–e) cross sections of the coatings; (f) fracture surface of the ceramic coatings; (g) fracture surface of the transition zone; (h) fracture surface of the metal– glass coatings. Fig. 8 Load–displacement curves of the ceramic–metallic glass gradient transition coatings and substrate. Fig. 9 Load–displacement curves of the ceramic–metallic glass and ceramic– NiCoCrAl gradient transition coatings [38]. Fig. 10 SEM image of indentations after the micro-hardness test: (a) ceramic coatings; (b) metal–glass coatings. Fig. 11 Crack evolution process under stress on the coating surface.

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Fig. 12 SEM image of the ceramic coating fracture surface (Fig. 7f): (a–e) ceramic coating fracture surfaces; (f) model of the fracture behaviour. Fig. 13 SEM image of the transition-zone fracture surface (Fig. 7g): (a, b) tensile fracture; (c) crack bypass; (d) pulling out and bridging; (e, f) model of the fracture behaviour. Fig. 14 Cross-sectional SEM images of the metal–glass coating fracture surface (Fig. 7h): (a, b) stepped fracture; (c) fracture mode of layer brittle fracture; (d) fracture mode of interlaminar crack extension; (e, f) model of fracture behaviour. Fig. 15 Model diagram of the laminar–columnar structure gradient transition effect.

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Table 1 Atmospheric plasma spraying parameters. Parameters

Step 1

Step 2

Spraying current, A

430

530

Standoff distance, mm

110

110

Powder feed voltage, V

6

8

Scanning rate of plasma torch, mm/s

4

4

N2 flow rate, L/h

2400

2000

Ar flow rate, L/h

2000

2000

H2 flow rate, L/h

5

10

N2 pressure, bar

11

11

Ar pressure, bar

10

10

H2 pressure, bar

11

11

29

Fig. 1

30

Ceramic coating (Al2O3-40 wt.% TiO2)

Gradient transition coating

Metal-glass coating

Metal-glass coating

Metal-glass coating

Fig.2

31

Micro-crack

Fig.3

32



Intensity(a.u.)



 Fe  TiO Gradient transition coating surface 2  Al2O3   Al TiO5   2

      





  Amorphous coating surface

10

20

30

40

50

2- (deg.)

Fig.4

33

60

70

80

Ceramic coating

Gradient transition coating

Ti

Al

Fe

Cr

Mo

B

O

Map Image

100μm

Fig.5

34

Al2O3-40 wt.% TiO2 Metal-glass layer

20μm

Ti

Al

Fe

Cr

Mo

B

O

Map Image

Fig.6

35

Ceramic coating Ceramic coating

Gradient coating Straigh

Gradient coating Zigzag

Metal-glass coating

Stepwise

Metal-glass coating

Ceramic coating

Gradient coating

Fig.7

36

Substrate Ceramic-metallic glassst gradient transition coatings

1000

3 stage

Substrate with coatings tangent Substrate tangent

800 st

900

Crack propagation

600 Load/N

Load/N

2 stage

400

850

800

st

1 stage

750

200

Yield stage 0.3

0.4

0.5

0.6

0.7

0.8

Displacement/mm

0 0.0

α

β

Ecoating > Esubstrate

0.2

0.4

Ecoating = tanα

0.6

Displacement/mm Fig.8

37

Esubstrate = tanβ

0.8

1.0

1000

Ceramic-metallic glass gradient transition coatings Ceramic-NiCoCrAl gradient transition coatings △F1 △F2

Load/N

800

NiCoCrAl

△F

NiCoCrAl

NiCoCrAl

600

400 △𝑭𝒎𝒆𝒕𝒂𝒍−𝒈𝒍𝒂𝒔𝒔 ≈ 𝟎 △FNiCoCrAl

200

△ 𝑭𝟏𝑵𝒊𝑪𝒐𝑪𝒓𝑨𝒍+△F2NiCoCrAl

△FNiCoCrAl >△ 𝑭𝒎𝒆𝒕𝒂𝒍−𝒈𝒍𝒂𝒔𝒔

0 0.0

0.2

0.4

0.6

Displacement/mm Fig.9

38

0.8

1.0

2463.8 HV0.3

3663.6 HV0.3

Fig.10

39

Fig.11

40

Fig.12

41

Fig.13

42

Model 1: stepped fracture

Step shape

Model 2: lamella fracture

Laminar

Fig.14

43

Crystalline fracture

Columnar

Pull out Tensile fracture Crack bypass

Laminar Step shape (interlaminar fracture)

Fig.15

44

Graphical abstract:

45