Micro-Mechanical Properties and Corrosion Resistance of Zr-Based Metallic Glass Matrix Composite Coatings Fabricated by Laser Cladding Technology

Abstract

Laser cladding with ultrafast cooling rates enables effective fabrication of metallic glass matrix composite (MGMC) coatings, significantly enhancing the hardness, corrosion resistance, and mechanical properties of metallic substrates. In this study, a multi-layer Zr₆₅Al_7.5Ni₁₀Cu_17.5 (at. %) MGMC coating was successfully fabricated by laser cladding technology. The effects of the region-dependent microstructural evolution on micro-mechanical properties and corrosion resistance were systematically investigated. The results indicated that the high impurity content of the powder feedstock promoted the crystallization of the coating during laser cladding. Moreover, coarse columnar crystals in the bottom region of the coating nucleated epitaxially at the coating/substrate interface and propagated along the thermal gradient parallel to the building direction, while dendritic crystals dominated the middle region under moderate thermal gradients. In the top region, fine dendritic and equiaxed crystals deposited in the amorphous matrix, due to the lowest thermal gradient and the highest cooling rate. Correspondingly, nanoindentation results revealed that the top region exhibited peak hardness (H), maximum elastic modulus (E), and optimal H/E ratio, exceeding values in both the bottom region and substrate. Simultaneously, the metallic glass matrix composite coating demonstrated significantly better corrosion resistance than the substrate due to its amorphous phase and protective passive film formation. This work advances amorphous solidification theory while expanding applications of metallic glasses in surface engineering.

1. Introduction

Metallic glasses (MGs), commonly termed amorphous alloys [1], have attracted significant research interest due to their unique mechanical, chemical, and physical properties [2,3,4]. Structurally, the absence of dislocations and grain boundaries in their disordered atomic arrangement enables most bulk metallic glasses (BMGs) to exhibit high strength, large elastic limits, superior corrosion resistance, and exceptional wear resistance compared to conventional crystalline alloys [5,6]. Owing to their wide supercooled liquid region (SLR), high glass-forming ability (GFA), and relatively low critical cooling rates, Zr-based MGs emerge as the most promising structural and functional materials among amorphous alloy systems [7,8]. Nevertheless, despite these advantages, the manufacturing limitations of conventional techniques, including copper mold casting, strip casting, and water quenching [9], restrict product dimensions and cooling rates, thereby impeding practical applications.

Owing to its rapid heating and cooling rates, laser cladding has emerged as a promising technique for overcoming the size limitations inherent in BMG production [10,11]. In this process, a high-energy laser beam melts pre-placed powder layers, enabling near-net-shape fabrication through sequential solidification. This approach achieves cooling rates sufficient for amorphous phase retention. Moreover, the laser cladding coating exhibits excellent metallurgical bonding with substrate materials. Nevertheless, repeated thermal cycling during multi-layer deposition induces structural relaxation and partial crystallization, yielding the metallic glass matrix composite (MGMC) with dual-phase amorphous-crystalline microstructures. The method remains technologically viable due to the extreme energy density and ultrafast thermal transients inherent to laser cladding [12].

As a surface modification technology, laser cladding has recently garnered significant research interest. This technique offers distinct advantages including geometric flexibility, protective coating capability, and process cleanliness [13]. Hu et al. [14] synthesized Zr_50.7Cu₂₈Ni₉Al_12.3 metallic glass matrix composite coatings via laser remelting with varied laser power. Their results confirmed that laser remelting promotes amorphous phase formation, significantly enhancing the corrosion resistance of MGMC coatings. Additionally, based on the point defect model, a hypothesis was proposed to elucidate the exceptional corrosion resistance mechanism in these coatings. Complementary, Song et al. [15] successfully fabricated Fe-based amorphous (Fe_41.5Co_12.2Cr_7.4Mo_37.3C_0.3B_0.5Y_0.4Al_0.4, at. %) coatings by laser cladding to investigate ultrasonic vibration (UV) effects on microstructural evolution. Their results demonstrated that UV treatment reduced the average columnar crystal length by ≈ 58% (from 25.26 ± 5.89 μm to 10.73 ± 3.91 μm) and increased amorphous content from 68.5% to 75.3% compared to non-UV processed coatings. This microstructural refinement and enhanced amorphous fraction significantly improved corrosion resistance. Similarly, Sohrabi et al. [16] deposited Zr_59.3Cu_28.8Al_10.4Nb_1.5 (at. %) amorphous coatings on aluminum substrates by laser cladding technology, identifying critical laser power thresholds for interfacial crack initiation. By optimizing two distinct parameter sets, they achieved crack-free coatings with higher than 99.5% density and maximized amorphous fraction, resulting in 20 times superior wear resistance relative to the substrate. Consequently, most previous studies have demonstrated that laser cladding amorphous coating is an effective strategy to enhance toughness, corrosion resistance and wear resistance of the metal materials. However, critical challenges remain in deep understanding the relationship between performance enhancement and the region-dependent, unpredictable microstructural characteristics within the coating.

This study aims to address the challenges of region-dependent microstructural evolution on mechanical properties in laser cladding coatings. Here, a Zr-based metallic glass matrix composite coating was successfully fabricated by laser cladding technology. The influence of the impurity of the powder feedstock on the phase composition and microstructure of the coating was studied. The microstructural evolution, micro-mechanical properties and electrochemical characteristics were systematically investigated. Meanwhile, the underlying enhancement mechanisms for both mechanical performance and corrosion resistance were scientifically revealed.

2. Material and Methods

2.1. Raw Materials

The 304L stainless steel plate was cut, polished and cleaned into 50 mm × 50 mm × 5 mm rectangular specimens, which were used as the substrate. In this present work, the Zr-based alloy ‘Inoue Glass’, known for its high glass-forming ability (GFA), was selected for laser cladding owing to its exceedingly low critical cooling rate of 1.5 K/s for amorphous formation [17]. Raw Zr, Al, Ni and Cu powders (supplied by Asia New Materials (Beijing) Co., Ltd., Beijing, China) were obtained via the high-pressure inert gas atomization method. Then these powders were weighed according to the stoichiometric ratio Zr₆₅Al_7.5Ni₁₀Cu_17.5 (at. %) in an argon-atmosphere glove box. A high-energy planetary ball mill (GN-2), using stainless steel vials and balls, was operated at 400 rpm with a ball-to-powder ratio of 10:1. The vials were filled with high purity argon gas to avoid oxidation. The raw powders were milled for a fixed time of 12 h. Eventually, the Zr₆₅Al_7.5Ni₁₀Cu_17.5 metallic glass matrix composite (abbreviated as Zr₆₅ MGMC) of cladding powders were obtained. The SEM image of Zr₆₅ MGMC powders shown in Figure 1a indicates that the powder particles are mostly smooth spheres, although some irregular particles and powder adhesion can also be observed. The average particle size of the raw powder particles is 29.1 μm, as shown in Figure 1b. The XRD patterns of the experiment raw powders are shown in Figure 1c. The results indicate that phases such as ZrO₂, Al₂O₃, and CuO form due to residual O₂ in the powder system during powder preparation.

2.2. Manufacturing Process and Parameters

Laser processing was conducted with a TQSL 1000-D01Nd: YAG laser (Beijing, China). In this work, the optical focus spot diameter of the laser beam was 1.5 mm, the scanning speed was 5 mm/s and the single laser pulse peak power was 400 W with pulse width of 6 ms. The hatch spacing was 1.2 mm. The volumetric energy density was calculated as 66.7 J/mm³ according to the equation $E={\displaystyle \frac{P}{vht}}$. Where the P is the laser power, v is the scanning speed, t is the layer thickness and h is the hatch spacing. The laser working voltage of 100 V and pulsed frequency was 33 Hz. The specimen surface was ground with 800 grit SiC paper and cleaned with acetone prior to the laser cladding. This experiment was carried out the pre-placed powder method with about l mm thickness powder bed onto the substrate surfaces homogeneously. Argon was served as the shielding gas to prevent alloy powder from the oxidation during laser processing. Before the next forming track, the previous track was ground with 800 grit SiC paper, and cleaned with acetone. Eventually, a five-layer MGMC coating with a total thickness of 1.26 mm was successfully produced. The cladding coating sample surfaces were polished with 800 grit SiC paper, then cleaned with pure ethanol and dried in air. Figure 2 shows the schematic diagram of the laser processing process, indicating the areas where electrochemical measurement and nanoindentation tests were conducted.

2.3. Characterization

The Zr₆₅ MGMC coating sample was cut along transverse section, then polished and etched with a corrosive agent consisting of 10 mL H₂O, 10 mL HNO₃, and 1 mL HF (in a 10:10:1 ratio). X-ray diffraction (XRD, TD-3500, Dandong, China) using Cu Kα irradiation was carried out for the phase composition in original powders and the MGMC coating with the 2θ range of 20–80°. Then the microstructure and distribution of chemical elements of the cladding coating were analyzed by JEOL scanning electron microscope (SEM, Zeiss GEMINI 500, Oberkochen, Germany) and energy dispersive X-ray spectroscopy (EDS). Additionally, transmission electron microscopy (TEM, JEM-2100F, Tokyo, Japan) was used to study the crystallographic feature of the matrix.

Prior to electrochemical measurement, the samples with the exposed surface area of 1 cm² were moistened, ground with 1200 grit silicon carbide paper, and ultrasonically cleaned in acetone. The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests of the coating sample were performed in a 3.5% sodium chloride solution (pH = 7) at room temperature (25 °C) using a CS350 electrochemical measurement system (CorrTest, Wuhan, China). The potentiodynamic polarization test was conducted in a three-electrode cell using a platinum net counter electrode and a saturated calomel reference electrode (SCE). The potential scan was initiated at −1.2 V (SCE), below the open-circuit potential (OCP), and scanned to 0 V (SCE) at a scanning rate of 0.5 mV/s. To achieve a stable reference potential for electrochemical tests, the OCPs of all samples were immersed in the 3.5% sodium chloride solution for 0.5 h. Electrochemical impedance spectroscopy measurements were conducted over a frequency range from 100 kHz to 10 mHz.

Nanoindentation testing was performed using an Agilent G200 TriboIndenter (Santa Clara, CA, USA) with a standard Berkovich diamond tip at room temperature. The indentation tests were conducted in a continuous stiffness mode at a constant loading/unloading rate of 1.0 mN/s. Ten indentations in each region were performed to verify the accuracy of the indentation data. The typical residual indentation morphology was further analyzed using a scanning electron microscope (SEM).

3. Results and Discussion

3.1. Phase Composition

The X-ray diffraction pattern of laser cladding coating is demonstrated in Figure 3. The pattern shows a broad diffraction peak at 2θ = 38°, which indicates the existence of the amorphous phase during laser processing. Superimposed on this broad peak are several sharp diffraction peaks, characterized as crystalline Zr, ZrO₂, Ni₁₀Zr₇, Al₂O₃, CuO, and Fe₂O₃ phases. Due to varying temperature gradients across different regions of the coating, these crystalline phases are more likely to form through structural relaxation and reheating effects. Additionally, dilution from the substrate introduces metallic elements (e.g., Fe) into the coating, causing the stoichiometric ratio of chemical elements in the cladding layer to deviate from the nominal Zr₆₅Al_7.5Ni₁₀Cu_17.5 composition. This deviation is evidenced by the formation of the Fe₂O₃ phase within the coating. Furthermore, the introduction of oxides or oxygen into the raw powders after milling promotes the crystallization of the coating during laser cladding, and then significantly influences the coating’s microstructure by altering the fluid flow dynamics within the molten pool [18].

3.2. Microstructure

To further characterize the microstructure, cross-section SEM images of the Zr₆₅ MGMC coating (Figure 4a–c) and a TEM bright-field image (Figure 4d) are presented. The coating exhibits a complex, region-dependent microstructure resulting from varying thermal gradients and cooling rates at different distances from the substrate. Figure 4a shows sound metallurgical bonding at the coating/substrate interface. High thermal gradients combined with substrate dilution/stirring effects at this interface promote crystalline phase formation [19]. Coarse columnar crystals nucleated epitaxially at the interface and grew upward along the thermal gradient through the deposition direction. As thermal gradients decreased, dendritic crystals formed in the middle region (Figure 4b), exhibiting microstructures significantly distinct from the bottom deposition zone. In the upper region, where thermal gradients were least steep and cooling rates highest, fine dendritic and equiaxed crystals (Figure 4c) formed within the amorphous matrix. Figure 4d confirms the amorphous nature of this matrix, displaying no discernible crystalline features and a characteristic broad halo ring in the diffraction pattern.

Consequently, both the temperature gradient (G) and the cooling rate (R) are critical factors governing the evolution of the microstructure in metal deposited parts. A higher temperature gradient combined with a lower cooling rate favors the formation of coarse columnar crystals. As the G/R ratio decreases, the grain structure tends to develop dendritic characteristics. Furthermore, a significant fine dendritic and equiaxed grain structure is likely to form in the upper regions of the deposited coating, owing to the lowest G/R ratio and superior heat dissipation conditions.

Figure 5 presents the elemental distribution obtained by line-scan EDS across the laser cladding coating and the substrate. During the deposition of the bottom layer, the laser beam energy generated a melt pool, forming a dilution zone at the interface. The composition of the deposited Zr-Al-Ni-Cu layer was altered by this dilution effect, inducing crystallization due to the diffusion of Fe from the substrate. In the middle and upper regions of the coating, the distributions of Zr, Al, Ni, and Cu closely matched the nominal composition of Zr₆₅Al_7.5Ni₁₀Cu_17.5, which facilitated the formation of an amorphous structure. However, the complex thermal cycles inherent in multi-layer laser processing, combined with the high impurity content of the raw powder, led to slight compositional fluctuations within the coating. Consequently, the combined effects of elemental redistribution, varying temperature gradients, and solidification conditions resulted in a regionally heterogeneous microstructure throughout the coating.

3.3. Nanoindentation Response

To gain a deep understanding of the effect of the heterogeneous microstructure, nanoindentation tests were conducted at the substrate and different regions within the coating. The hardness and modulus versus displacement curves are presented in Figure 6. During laser processing, distinct thermal histories in different regions led to unique microstructural characteristics. Figure 6 shows that both hardness and modulus are highest in the top region, followed by the bottom region, whereas the substrate exhibits the lowest values. These results confirm that the laser cladding coating possesses higher hardness and modulus than the substrate, attributable to its composite microstructure consisting of an amorphous matrix and fine crystalline phases. Furthermore, when internal stresses propagate into the coating, the amorphous matrix’s significant plastic strain capacity facilitates a stress redistribution effect. This, combined with the constraints imposed by the crystalline phases, leads to work hardening and consequently higher hardness.

The hardness (H), elastic modulus (E), and H/E ratio obtained from Hardness/Modulus-displacement curves using the Oliver-Pharr method are summarized in

Table 1. Hardness (H), elastic modulus (E), H/E ratio of the samples.

Region	Hardness, H (GPa)	Elastic Modulus, E (GPa)	H/E Ratio
Top region	7.6 ± 0.5	133 ± 5	0.057 ± 0.004
Bottom region	7.5 ± 0.5	132 ± 5	0.057 ± 0.005
Substrate	7.1 ± 0.5	162 ± 5	0.044 ± 0.005

. The H/E ratio commonly indicates the elastic recovery of a material [20]. Obviously, the top region presents the highest hardness and elastic modulus of 7.6 ± 0.5 GPa and 133 ± 5 GPa, respectively. The coating exhibits the higher H/E ratio than the substrate, suggesting a high degree of elastic recovery. During laser processing, significant crystallization occurred within the coating. This process reduced free volume and formed thermodynamically stable intermetallic crystalline phases (as confirmed by XRD in Figure 3), resulting in increased hardness and elastic modulus compared to the substrate.

As shown in Figure 7, typical residual indentation morphologies were observed using SEM. Consistent with findings reported in [21], a low loading rate (0.04 mN/s) promotes more pronounced serrated flow, whereas a high loading rate (5 mN/s) suppresses it. In the present study, an intermediate loading rate (1.0 mN/s) was employed, resulting in a high frequency of serrated flow events. Previous studies attributed these discrete serrations to the nucleation and propagation of individual shear bands [22]. Our results demonstrate that numerous serrated flow events occurred during indentation, corresponding to the activation of multiple shear bands (indicated by yellow arrows in Figure 7, top and bottom images). This high density of shear band activity provides further evidence for the coating enhanced resistance to plastic deformation, explaining its higher hardness and elastic modulus compared to the substrate.

3.4. Corrosion Resistance

Figure 8 presents the potentiodynamic polarization plots of various regions in the coating and the substrate. Corrosion current density (icorr), which is correlated with corrosion potential (Ecorr), serves as a key indicator for evaluating a material’s corrosion-resistance performance [23]. In this study, the Ecorr and icorr values for both the coating and substrate were determined using the Tafel extrapolation method [24], with the results summarized in

Table 2. Corrosion potentials (Ecorr) and current density (icorr) of the laser cladding coating.

	Ecorr (mV)	icorr (A/cm2)
Top region	$-$395	1.1512 $\times$ 10−7
Bottom region	$-$421	3.2878 $\times$ 10−7
Substrate	$-$439	6.8217 $\times$ 10−7

. Typically, superior corrosion resistance, characterized by higher chemical stability, lower corrosion tendency, and lower corrosion rate, is indicated by a lower icorr value and a higher Ecorr value.

As shown, the top region has the lowest icorr value around 1.1512 $\times$ 10⁻⁷ A/cm² and the most noble Ecorr value around −395 mV compared with the bottom region and substrate. The top region exhibits the best corrosion resistance, attributed to both the fine dendritic, equiaxed crystals and the amorphous matrix. Han et al. [25] indicated that amorphous phase formation plays a beneficial role in improving corrosion resistance. The bottom region, characterized by coarse columnar crystals, exhibits weaker corrosion resistance, followed by the substrate which shows the weakest corrosion resistance. Figure 8 shows a passive region in the potentiodynamic polarization plots of the deposited coating, occurring in the potential range from −0.3 to −0.2 V, which is also an important reason that the coating exhibits excellent corrosion resistance. The formation of a passive film can prevent the underlying alloy from further corrosion by acting as a barrier with low electrical conductivity, thereby improving the corrosion resistance of the coating. Meanwhile, the pitting potential (E_pit) is defined as the critical potential at which the passivation film ruptures. Higher E_pit values indicate superior pitting resistance [23]. As revealed in Figure 8, the top region exhibits significantly enhanced pitting resistance compared to both the bottom region and substrate. In this work, according to the previous studies [14,23], the passive film of the cladded coating may be mainly consisted of ZrO₂ and Al₂O₃. This is also consistent with the XRD results in Figure 3. Consequently, the formation of the passive film in the Zr₆₅ MGMC coating is a key factor resulting in its superior corrosion resistance compared to the substrate.

Figure 9 shows the Nyquist plots for the top region, bottom region of the coating, and the substrate. Electrochemical impedance spectroscopy (EIS) was measured to evaluate the electrochemical behavior of the samples in a 3.5 wt.% sodium chloride solution at open-circuit potential. The radius of the capacitive loop in the Nyquist plot correlates with the material’s corrosion resistance [26]. For all samples, a high-frequency capacitive loop is observed, which is associated with the material’s properties. As shown, the capacitive loop radius is largest for the top region, intermediate for the bottom region, and smallest for the substrate. This indicates that the top region exhibits the best corrosion resistance, a finding consistent with the conclusion drawn from the previous potentiodynamic polarization analysis. The superior corrosion resistance observed in the coating (particularly the top region) is attributed to the presence of the amorphous phase and effective passivation behavior.

To quantify EIS results and probe the physicochemical behavior of coating corrosion, the impedance spectra were modeled using equivalent circuits, as presented in Figure 10. The equivalent circuit consists of the solution resistance, R_Ω, representing ohmic losses in the electrolyte; the charge transfer resistance, R_ct, characterizing the kinetic barrier to electrochemical reactions at the interface; and the capacitance connected with the layers and solution, C_d, accounting for the capacitive behavior associated with the electrical double layer and surface inhomogeneities. Corrosion by the sodium chloride solution initiates at the coating surface. The passive film formed within the amorphous region provides a barrier, significantly delaying the corrosion of the underlying coating even after prolonged immersion times. Eventually, breakdown or degradation of the passive film can allow corrosion to propagate to the substrate.

4. Conclusions

In this work, a multi-layer Zr₆₅Al_7.5Ni₁₀Cu_17.5 metallic glass matrix composite coating was successfully fabricated using laser cladding technology. The microstructure evolution, micro-mechanical properties and electrochemical characteristics were systematically investigated. The key conclusions can be summarized as follows:

(1)

The composite coating formed a metallurgical bond with the substrate and exhibited negligible porosity without crack formation. However, the high impurity content of the raw powder promoted the crystallization of the coating during laser cladding.

(2)

Microstructural evolution across the coating exhibits significant regional heterogeneity. Coarse columnar crystals in the bottom region nucleate epitaxially at the coating/substrate interface and propagate along the thermal gradient parallel to the building direction, while dendritic structures dominate the middle region under moderate thermal gradients. Ultimately, fine dendritic and equiaxed crystals form within the amorphous matrix in the top region, attributable to the lowest thermal gradient and the highest cooling rate during laser processing.

(3)

The microstructure characteristics in the top region of the coating contribute to superior hardness, elastic modulus, and H/E ratio compared to both the bottom region and substrate. Enhanced micro-mechanical properties in the coating originate from the initiation and propagation of multiple shear bands.

(4)

The metallic glass matrix composite coating demonstrates significantly better corrosion resistance than the substrate due to its amorphous phase and protective passive film formation.