Excellent thermal shock resistance of NiCrAlY coatings on copper substrate via laser cladding

doi:10.1016/j.jmst.2022.05.011

Abstract

Abstract:

Coatings are widely used to protect substrates in extreme thermal environments (e.g., arc heaters), and thermal shock resistance is a crucial parameter for the coatings, which requires tight interlayer bonding between coatings and substrates. In this work, NiCrAlY coatings were highly required for the pure copper substrate to restrict the electric arc in arc heaters. To overcome the bonding difficulty of coating on the copper surface, the NiCrAlY coatings were prepared by two laser cladding methods: conventional laser cladding (CLC) and high-speed laser cladding (HSLC). The microstructure, composition, and thermal shock resistance of NiCrAlY cladding layers prepared by both methods were investigated. Benefitting from the high cooling rate and high energy density, the HSLC-layer has better composition uniformity and tighter interlayer bonding than the CLC-layer, achieving a 30%-45% improvement in thermal cycling lifetime. Besides, the NiCrAlY layers prepared on copper substrate by both laser cladding methods exhibit 3-10 times better thermal shock resistance than those NiCrAlY layers prepared by conventional spraying methods. It further confirms the great effects of metallurgical bonding and composition uniformity on the thermal shock resistance of coatings. The NiCrAlY layers fail in the form of internal cracking and interface peeling, and the corresponding failure mechanism is discussed.

Key words: Laser cladding, NiCrAlY, Thermal shock resistance, Microstructure

Mingyu Gao, Shunchao Li, Weimian Guan, Hongbin Xie, Xiaoxiang Wang, Jiabin Liu, Hongtao Wang. Excellent thermal shock resistance of NiCrAlY coatings on copper substrate via laser cladding[J]. J. Mater. Sci. Technol., 2022, 130: 93-102.

Figures/Tables 15

Table 1. Chemical composition of the NiCrAlY powder (at.%).

Ni	Cr	Al	Y
70.3	19.5	9.7	0.5

Fig. 1. Scanning electron microscopic image of the NiCrAlY powder.

Fig. 2. Schematic diagrams of (a) HSLC and (b) CLC.

Table 2. Typical deposition parameters of the two cladding modes.

Type	Total laser power (W)	Beam diameter (mm)	Powder feed rate (g/min)	Scanning speed (mm/s)	Overlap ratio (%)
HSLC	3925	1.5	4.8	90	75
CLC	2310	3.0	5.5	10	30

Fig. 3. 3-D morphologies of the (a) HSLC- layer and (b) CLC-layer.

Fig. 4. Cross-sectional optical micrographs of the (a) HSLC cladding layer and the (b) CLC cladding layer. (c) and (d) correspond to the magnified images of areas A and B in (a) and (b), respectively.

Fig. 5. SEM images of the cladding layers: Cross-sectional microstructure of (a) HSLC-layer and (d) CLC-layer; near-substrate horizontal microstructure of (b) HSLC-layer and (e) CLC-layer; near-surface horizontal microstructure of (c) HSLC-layer and (f) CLC-layer.

Fig. 6. (a, b) XRD patterns of the HSLC-layer and CLC-layer. EDS mapping of (c) HSLC-layer and (d) CLC-layer.

Fig. 7. Cross-sectional SEM images of (a, b) HSLC-layer and (c) CLC-layer. The line scans of element distribution along the yellow line.

Fig. 8. Thermal conductivity and microhardness along the build direction of (a) HSLC and (b) CLC.

Fig. 9. Images of the NiCrAlY samples after thermal shock tests at 600 °C and 800 °C: HSLC-layers (a, c); CLC-layers (b, d).

Fig. 10. Comparison of the thermal cycling lifetime of NiCrAlY coatings prepared on copper substrate by different methods: APS [31], HVOF [32], CLC, and HSLC. More than 20% of the spalled area and cracked was defined as the criterion for the coating failure [24].

Fig. 11. Cross-sectional microstructure after thermal shock failure: (a) CLC-layer and (b) HSLC-layer. The insets (a) and (b) display the original cross-sectional images of the claddings. The EDS mappings correspond to the area circled by the blue dashed rectangle (c).

Fig. 12. (a) Cross-sectional image of the CLC-layer after thermal shock failure. (b) Magnified image of area A in (a). (c) Microcracks extend along the dendrite interstices.

Fig. 13. Schematic diagram of failure mechanism for (a) HSLC-layer and (b) CLC-layer in thermal shock process.

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