Effect of chromium addition on the microstructure and mechanical properties of laser cladding Ti-Al-SiC composite coatings

Abstract

Chromium (Cr) critically enhances interfacial compatibility and mechanical properties of titanium (Ti) -based coatings, however, the specific effects of Cr content on microstructural evolution remain underexplored. In this study, Ti-Al-SiC-Cr_x (x = 0, 5, 10, 15) coatings were fabricated on Ti-6Al-4V (TC4) titanium alloy via laser cladding. The phase composition of the coating/annealed TC4 was determined by X-ray diffraction (XRD). The microstructure was observed using scanning electron microscopy (SEM), and the grain orientation was analyzed by electron backscatter diffraction (EBSD) attached to the SEM. The effects of Cr content on the microstructural evolution and grain characteristics of the coatings were investigated, and the resultant variations in mechanical properties were further analyzed. Results indicate that increasing Cr content promotes Ti₄Cr/Cr₂AlC precipitation, reduces Ti₈C₅ and increases Ti₅Si₃. Notably, the orientation strength of Ti₅Si₃ is enhanced, and the grain size of the coating is refined. Both the TC4 substrate and the laser-cladding coatings exhibit strain rate strengthening behavior. Crucially, yield strength, peak stress, and microhardness positively correlate with Cr content. At identical indentation depth, maximum load, indentation hardness, and Young’s modulus progressively increase with Cr content, exceeding TC4 substrate values. Although friction coefficients rise with Cr content, the coating with 5 wt% Cr (Cr₅) demonstrates minimal wear weight loss. This study highlights the potential of laser cladding Ti-Al-SiC-Cr_x coatings for aerospace applications, offering a unique combination of refined microstructure and superior mechanical performance.

Introduction

Titanium and its alloys, renowned for their exceptional specific strength, superior impact toughness, and excellent corrosion resistance, serve as the primary structural materials for cold-end components, particularly compressor blades, in aero-engines^1,2,3. Despite these advantages, titanium and its alloys are hindered by several limitations, including low surface hardness, low work-hardening rate, and inadequate wear resistance^4,5,6. To ensure the operational safety of these components and mitigate the risk of failure, titanium-based coatings are frequently employed in engineering applications to enhance the overall mechanical properties of titanium alloy surfaces^7,8. Given the pronounced sensitivity of TC4 titanium alloy to heat input, laser cladding emerges as an optimal surface enhancement strategy. This technique affords a higher energy density and cooling rate compared to electric arc or plasma cladding, leading to a finer microstructure, a narrower heat-affected zone, and minimal substrate deformation. Moreover, whereas electron beam cladding necessitates a vacuum chamber, laser cladding operates in a non-vacuum environment, granting superior flexibility for the in-situ treatment of complex geometries^9,10,11.

To mitigate the impact of heterogeneous interfaces, laser cladding was employed to fabricate Ti-Al based coatings on TC4 titanium alloy substrates, with the objective of enhancing surface hardness and wear resistance¹². The influence of ceramic powder content on the microstructure, hardness, and wear resistance of the coatings was systematically investigated by varying the ceramic powder concentration within the Ti-Al mixed powder system. For instance, the addition of SiC facilitated the precipitation of reinforcing phases, namely Ti₅Si₃ and TiC, within the coatings, thereby enhancing material hardness and reducing the friction coefficient^13,14. Within the range of 0–6%, the progressive increase in TiC content promoted the formation of Ti₃AlC₂, where TiC served as a reinforcing phase, consequently improving the coating’s hardness and wear resistance¹⁵.

Chromium (Cr) is a widely utilized alloying element in wear-resistant steels due to its significant role in carbide formation and modification. Extensive research has demonstrated that Cr not only promotes carbide formation but also alters the microstructure, morphology, and properties of these carbides, thereby enhancing the wear resistance of coatings. To systematically evaluate the influence of Cr content on coating performance, Fe-Cr_x-B coatings were fabricated using laser cladding technology, and experimental results revealed a positive correlation between Cr content and coating hardness, with the optimal performance achieved at 12 wt% Cr content¹⁶. The enhancement of wear resistance through Cr addition has been well-documented in the literature. Particularly, Cr-containing intermetallic compound coatings have demonstrated exceptional properties, including high hardness, refined microstructure, and robust intermetallic atomic bonding, which collectively contribute to their excellent wear resistance under dry sliding wear test conditions^17,18.

To further elucidate the influence of Cr on the comprehensive performance of laser additive manufactured components, the strategic design of V/Cr transition layers effectively suppresses the formation of brittle intermetallic compounds at the interface between titanium alloy substrates (e.g., Ti-6Al-4V) and nickel-based coatings, elucidating the regulatory mechanisms of transition layers on interfacial microstructure evolution and mechanical performance enhancement¹⁹. In Ni-Cr-Ti-B₄C composite coatings fabricated on Ti-Al intermetallic and Q235 steel substrates, laser-specific energy (Es) demonstrates precise control over ceramic phase distribution, resulting in a 40–60% improvement in coating hardness and significantly enhanced wear resistance²⁰. Cr exhibits dual-phase modulation capabilities: In Ti-xCr graded systems, β-solution heat treatment induces multi-phase transformations (e.g., α→β→α’ martensite), enabling tailored adjustment of Young’s modulus²¹, while in Ni-Ti-xCr coatings, increased Cr content drives a transition from columnar to equiaxed grain structures and promotes the formation of continuous Cr-rich intermetallic strengthening phases (e.g., Cr₂Ti and Cr₃Ni₂)²². For nickel-based superalloys, optimized Cr/Mo ratios combined with solution-aging treatments regulate γ-dendritic morphology and facilitate homogeneous precipitation of γ”-strengthening phases (volume fraction > 35%), offering innovative strategies for high-temperature component optimization²³. These findings establish a theoretical foundation for laser additive manufacturing of high-performance metal matrix composites, emphasizing the critical synergy among compositional design, process parameters, and post-processing strategies in achieving tailorable material properties.

In summary, Cr plays a pivotal role in enhancing the mechanical properties of titanium-based coatings. To clarify the influence of Cr on the mechanical properties and wear resistance of coatings, and to reveal the strengthening and toughening mechanism of coatings, laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings were taken as objects to comprehensively examines the influence of Cr content on microstructural evolution, microhardness, dynamic mechanical properties, and wear resistance characteristics. These findings provide substantial experimental data and theoretical insights for optimizing surface hardness, strength, and wear resistance in titanium alloy applications.

Materials and methods

Materials preparation

Ti, Al, SiC and Cr powders (Chengdu Huayin Powder Technology Co., Ltd.) were selected as the raw materials for laser cladding of titanium-based coatings, and the microscopic morphology of the powders was shown in Fig. 1a–d. Ti and Al powders exhibited a spherical morphology, with high fluidity, while SiC and Cr powders exhibited an irregular block shape, with poor flowability. In order to minimize the effect of powder fluidity on the laser cladding process, the pre-powder method was used for cladding. A particle size analyzer (Hydro 2000SM, Malvern Panalytical) was employed to measure the particle sizes of the powders. The particle size distributions for Ti, SiC, and Cr powders were presented in Fig. 1e–g, respectively. The Ti powder demonstrated D10, D50, and D90 values of 19.26 μm, 36.22 μm, and 63.05 μm, respectively. The SiC powder exhibited corresponding values of 70.65 μm, 101.06 μm, and 137.78 μm, respectively, and D10, D50, and D90 values of the Cr powder were 20.03 μm, 38.37 μm, and 65.65 μm, respectively. All powder particle sizes conform to a lognormal distribution. Additionally, the Al powder exhibits a particle size range of 50 to 100 μm and possesses a purity level of 99.9%. Since Al is a highly reactive metal and may react with ions dissolved in water to produce hydrogen. Due to experimental constraints, its particle size distribution was not tested.

TC4 titanium alloy (with dimensions of 60 mm × 60 mm × 20 mm) was used as the cladding substrate. The laser cladding powders were prepared using Ti, Al, SiC, and Cr as raw materials according to the compositions listed in Table 1. As shown in the table, the relative proportions of Ti, Al, and SiC were maintained constant, while the Cr content in the powder mixture was varied at 0 wt%, 5 wt%, 10 wt%, and 15 wt%. These compositions were designated as Cr₀, Cr₅, Cr₁₀, and Cr₁₅, respectively. This approach ensured that any effects observed could be attributed solely to the variation in Cr content, thereby eliminating potential uncertainties arising from changes in the proportions among Ti, Al, and SiC. The metal powder was homogenized using a mixer (JHX100L, Zhengzhou Jinhe Equipment Manufacturing Co., Ltd.) before cladding, and the mixing parameters were set at a speed of 20 r/min for a duration of 1 h. After the powder was thoroughly mixed, it was placed in an atmospheric furnace (HMX 1600-30, manufactured by Shanghai Haoyue Electric Furnace Technology Co., Ltd.) and dried at 100 °C for 2 h.

Fig. 1

Micro-morphology and particle size distribution of Ti, Al, SiC and Cr powders. (a) Ti. (b) Al. (c) SiC. (d) Cr. (e) Ti particle size distribution. (f) SiC particle size distribution. (g) Cr particle size distribution.

Table 1 Mixing ratios of powders (wt%).

The substrate surface was sequentially polished with sandpaper of 300, 600, 1200, and 2000 grit sizes and subsequently cleansed with anhydrous ethanol to remove impurities. Fiber laser cladding equipment (RFL-C6000, Wuhan Raycus Fiber Laser Technology Co., Ltd.) was used for laser cladding, S-type reciprocating scan pattern and preset powder feeding method were adopted. (Fig. 2a,b). Four different powder ratios were utilized to prepare four coatings with a thickness of 1 mm and another set of four coatings with a thickness of 5 mm (multiple powder claddings). 1-mm-thick coatings were produced by single-layer cladding, while 5-mm-thick coatings were fabricated in three layers with an inter-track overlap ratio of 50%. The laser power, scanning speed, and spot diameter during cladding were set to 1800 W, 10 mm/s, and 3 mm, respectively. In order to inhibit oxidation of the material, 99% pure argon gas was used as a protective gas. To facilitate subsequent microstructure characterization and mechanical property evaluations, the coatings were precisely cut by wire electrical discharge machining (WEDM) according to the size of the test specimens, as illustrated in Fig. 2c, d. The square-shaped specimens were used for XRD, SEM, EBSD, nanoindentation, friction and wear tests, and the cylindrical specimens were used for dynamic mechanical property tests.

Fig. 2

Schematic diagram of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) specimen (in mm). (a) Experimental setup of laser cladding. (b) Scanning path. (c) WEDM wire-cutting sampling. (d) Specimen size.

Characterization and testing methods

The coating materials with four powder ratios were characterized by XRD, SEM and EBSD to analyze the influence of Cr content on the microstructure and crystal characteristics of the materials. Then the mechanical property tests were conducted on four types of coating materials to investigate the effect of microstructure changes on the mechanical properties of the coatings. The specific characterization and testing methods are as follows:

The coatings were polished with argon ions. After polishing, the specimen surfaces were etched with a solution of HF: HNO₃: H₂O in a ratio of 3:6:91 (Volume ratio). An X-ray diffractometer (D8 Advance, Bruker) was used for the physical phase analysis of the coating, and the test was performed in continuous scanning mode with a voltage of 40 KV, a current of 15 mA, a diffraction angle of 10–90°, a scanning speed of 8°/min, and a step size of 0.2°. The surface micromorphology of the coatings was observed using a scanning electron microscope (Sigma 300, ZEISS), and the elemental distribution of the coatings was analyzed using its equipped energy disperse spectrometer (EDS). The crystal structure of the coating was characterized by the electron backscatter diffraction method and EBSD probe (C-Nano EBSD, Oxford).

The mechanical properties of the coatings were tested by Vickers hardness tester (402 MVD, Vickers), nanoindentation tester (U9820A Nano Indenter G200, Agilent), split Hopkinson pressure bar (SHPB) device (Leway Technology Φ14.5, Luoyang Leway Technology Co., Ltd.), and friction testing machine (UMT 2MT, USA-CETR). The surface morphology of the wear scars on the coatings was observed by three-dimensional profile tester (VR 3200, Keyence). Vickers hardness test parameters included a load of 200 g and a duration of 10 s. Nanoindentation test parameters included static displacement mode, a maximum indentation depth of 1 μm, and a maximum load duration of 10 s. Dynamic compression mechanical properties test parameters included loading strain rates of 120 s⁻¹, 180 s⁻¹, and 300 s⁻¹. Friction and wear test parameters included a ball-block type friction pair, a Si₃N₄ ball with a diameter of 6 mm as the friction counterpart.

Results and discussion

Phase identification

Figure 3 illustrates the XRD patterns of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings. As shown in the figure, the cladding coating phases mainly include ceramic phase compounds such as Ti₅Si₃, TiC, Ti₈C₅ and Cr₂AlC, as well as intermetallic compounds such as Ti₃Al and Ti₄Cr. Ti₅Si₃, Ti₃Al, TiC and Ti₈C₅ characteristic peaks are present in the Cr₀ coating, and intermetallic compounds Al₂Cr, Ti₄Cr and ceramic compound Cr₂AlC characteristic peaks are newly appeared in the Cr₅ coating. Compared to the Cr₅ coating, the characteristic peaks of the intermetallic compound Al₂Cr disappear in the Cr₁₀ coating, and the characteristic peaks of the intermetallic compounds Ti₄Cr and Al₂Cr disappear in the Cr₁₅ coating. The characteristic peaks of SiC were not seen in any of the Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings, which may be due to the in-situ formation of C and Ti that are dissociated from SiC. In addition, due to the lowest Gibbs free energy of the Ti₅Si₃ hard phase during laser cladding²⁴, the Ti element reacts preferentially with the Si element to form ceramic phase compounds. The chemical reaction formulas involving Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) cladding coatings during the melt pool reaction are as follows:

$$3{\text{Ti}} + {\text{Al}} \to {\text{Ti}}_{3} {\text{Al}}$$

(1)

$$2{\text{Al}} + {\text{Cr}} \to {\text{Al}}_{2} {\text{Cr}}$$

(2)

$$4{\text{Ti}} + {\text{Cr}} \to {\text{Ti}}_{4} {\text{Cr}}$$

(3)

$$8{\text{Ti}} + 3{\text{SiC}} \to {\text{Ti}}_{5} {\text{Si}}_{3} + 3{\text{TiC}}$$

(4)

$$2{\text{Ti}} + 2{\text{SiC}} + {\text{Cr}} \to {\text{Si}}_{2} {\text{Cr}} + 2{\text{TiC}}$$

(5)

$$5{\text{Ti}} + 3{\text{Al}} + 3{\text{SiC}} + 6{\text{Cr}} \to 3{\text{Cr}}_{2} {\text{AlC}} + {\text{Ti}}_{5} {\text{Si}}_{3}$$

(6)

Fig. 3

XRD patterns of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings.

Microstructural characterization

Metallographic analysis

Figure 4 illustrates the macroscopic cross-sectional morphology of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings. As shown in Fig. 4a–d, the thicknesses of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings are 1188 μm, 1186 μm, 1175 μm, and 1158 μm, respectively, the cross-section of cladding coating do not show obvious defects. However, the high energy in the laser cladding process leads to sublimation of a small portion of Ti, Al and SiC powders, and the sublimated gases fail to completely evaporate from the molten metal, so that pores would be formed inside the coatings as the molten metal solidified. Additionally, with a laser power of 1800 W, the peak temperature of the melt pool can reach 2000–2500 ℃. Under this high-energy input, partial decomposition of SiC occurs, accompanied by localized ablation that generates pores in the coating structure. By observing the interfacial position between the coating and the TC4 substrate, a good metallurgical bond exists between the two.

Fig. 4

Macroscopic cross-sectional morphology of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings. (a) Ti-Al-SiC-Cr₀. (b) Ti-Al-SiC-Cr₅. (c) Ti-Al-SiC-Cr₁₀. (d) Ti-Al-SiC-Cr₁₅.

Figure 5a-h illustrates the surface micromorphology of laser cladding coated Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings. Table 2 shows the EDS analysis results corresponding to the labelled regions in Fig. 5e-h. Taking Fig. 5e as an example, points A1, B1, and C1 mark EDS analysis locations. All points are situated at the center of single-phase regions (away from grain or phase boundaries) to ensure that the EDS data reflect solely the composition of the target phase. A1 corresponds to the dendritic/blocky hard phase, B1 to the continuous matrix phase (i.e., the network-like phase), and C1 to the ellipsoidal phase. The phase types were determined by combining the microchemical composition obtained via EDS point analysis with phase identification results from XRD. For instance, at point C3 in the dendritic-blocky region of Cr₁₀ coating, the Ti/Cr atomic ratio was 4.86:1. Cross-referencing with Ti₄Cr peaks in Fig. 3 confirmed this phase as Ti₄Cr intermetallic. On this basis, combining Fig. 5a–d; Table 2, it can be seen that the microstructure of the coatings mainly consists of ellipsoidal and long block phases, and Cr₅ and Cr₁₀ show similar microstructures including ellipsoidal dendritic phase (Ti₈C₅), long block phase (Ti₅Si₃), and matrix phase (Ti₃Al)²⁵. From Fig. 5a, it can be seen that the size of ellipsoidal Ti₈C₅ is about 2–4 μm with a relatively uniform distribution, while the Ti₃Al matrix phase exhibits a reticular structure. Ellipsoidal Ti₈C₅ with a size of about 3–5 μm is observed in Fig. 5b, and the size of the ellipsoidal particles is obviously increased compared to that in Fig. 5a. More long block phases Ti₅Si₃ are also observed in the coating, and Ti₄Cr appears in the Ti₃Al matrix. Compared with Fig. 5b, the size of long block Ti₅Si₃ in Fig. 5c increases, the proportion of ellipsoidal Ti₈C₅ decreases, and Ti₈C₅ aggregates around the Ti₅Si₃ structure to form a dendritic structure. In Fig. 5d, the ellipsoidal Ti₈C₅ is unevenly distributed, the size of Ti₅Si₃ in the long block phase decreases, and a continuous layered Cr₂AlC phase is also observed. TiC agglomerates with a size of about 15 μm are observed in all four coatings.

Fig. 5

Micro surface morphology of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings. (a) Ti-Al-SiC-Cr₀. (b) Ti-Al-SiC-Cr₅. (c) Ti-Al-SiC-Cr₁₀. (d) Ti-Al-SiC-Cr₁₅.

Table 2 Chemical composition of marked points in the coatings (Fig. 5e–h).

EBSD analysis

Figure 6 illustrates the crystal orientation and both pole figures and inverse pole figures of Ti-Al-SiC-Cr₅ and Ti-Al-SiC-Cr₁₅ coatings. The phase identification shows that the Cr₅ coating consists of 47.3% Ti₈C₅ and 37% Ti₅Si₃, the Cr₁₅ coating contains 27.3% Ti₈C₅ and 52.7% Ti₅Si₃, and the content of Ti₈C₅ and Ti₅Si₃ varies greatly between the two coatings. Figure 6a, b illustrate the Ti₈C₅, Ti₅Si₃ and Cr₂AlC orientations in Cr₅ and Cr₁₅, with colours distinguishing grain orientations. Comparing the two figures, it is found that the intergrain boundaries of the Cr₅ and Cr₁₅ coatings are clearly defined. The Cr content significantly influences the crystal orientation within these coatings, and the Cr₅ coating exhibits a widely dispersed crystal orientation across the observation area. In contrast, the Cr₁₅ coating demonstrates a more uniform orientation among adjacent grains, with obvious texture characteristics.

Fig. 6

EBSD images of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings. (a,b) are the crystal orientation figures of Cr₅ and Cr₁₅, (c,d) are pole figures and inverse pole figures of Ti₈C₅ in Cr₅ and Cr₁₅. (e,f) are pole figures and inverse pole figures of Ti₅Si₃ in Cr₅ and Cr₁₅.

Figure 6c, d illustrate the pole figure and inverse pole figure of Ti₈C₅ in Cr₅ and Cr₁₅ coatings, similarly, Fig. 6e, f show the pole figure and inverse pole figure of Ti₅Si₃ in Cr₅ and Cr₁₅ coatings. Table 3 shows the values of the crystal orientation and pole density values of Ti₈C₅ and Ti₅Si₃ in Cr₅ and Cr₁₅ coatings. The phase identification process for Ti₈C₅ is exemplified as follows. Initially, non-stoichiometric titanium carbide Ti₈C₅ (tetragonal crystal system, space group P4/mbm) was preliminarily identified in the coating via XRD (Fig. 3) as a candidate phase. Subsequently, based on SEM morphological observations (Fig. 5), typical blocky structural regions were identified to avoid interference from phase boundaries, and EBSD analysis was conducted within these selected areas. The phase was identified as Ti₈C through geometric matching of the acquired Kikuchi diffraction patterns with crystallographic databases. A positive identification was confirmed when the confidence index (CI) exceeded 0.2 and the geometric characteristics were distinctly different from those of similar phases such as cubic TiC. As shown in Fig. 6; Table 3, compared to the Cr₅ coatings, the values of the orientation strength of Ti₈C₅ in the Cr₁₅ coating slightly decreases, but the value of the orientation strength of Ti₅Si₃ is significantly increases, indicating that the strength of crystal orientation increases with the increase of Cr content. Attributed to the orderliness and orientation of the grain arrangement, the material is able to exhibit higher strength and hardness when the strength of the preferential orientation is high.

Table 3 Preferential orientation and pole density values of Ti₈C₅ and Ti₅Si₃ in coatings (Fig. 6c-f).

Figure 7a illustrates the statistics of low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs) of Cr₅ and Cr₁₅ coatings. As shown in the figure, the proportion of HAGBs and LAGBs in Cr₅ coating is 92.37% and 7.63%, respectively, and the Cr₁₅ coating exhibits a marked rise in the proportion of LAGBs to 16.59%. This observation indicates that as the Cr content increases, there is a corresponding increase in the proportion of LAGBs.

Fig. 7

Statistical diagram of grain boundaries and grain size. (a) LAGBs and HAGBs of Cr₅ and Cr₁₅. (b) Grain size of Cr₅ and Cr₁₅.

Figure 7b shows the grain size statistics of Cr₅ and Cr₁₅ coatings. The statistical results indicate that the average grain size decreased from 3.3 μm for the Cr₅ coating to 1.4 μm for the Cr₁₅ coating. Compared with the Cr₅ coating, the percentage of grains with grain size less than 2 μm increased from 50.96% to 65.13% for the Cr₁₅ coating, the percentage of grains with size 2–3 μm remains relatively unchanged, and the percentage of grains with size 3–10 μm decreased. The gradual refinement of the coating grains with increasing Cr content may be due to the fact that a large amount of Cr in the Cr₁₅ coating can increase the number of nucleation sites and improve the nucleation rate. In addition, the carbides formed by Cr and C provides more heterogeneous nucleation sites for the generation of other phases. In general, the refined grains can lead to a further increase in material strength and hardness²⁶.

Laser cladding pool model

The evolution of material phases in the molten pool during the laser cladding process was mapped through the combination of material phase identification, surface morphology analysis and crystal structure characterization, as shown in Fig. 8. During the laser cladding process, Ti, Al, SiC and Cr powders are transformed into Ti, Al, Si, C and Cr active elements under the high temperature of the high-energy laser beam, as the laser heat input increases, the molten pool is gradually formed, and the elements in the molten pool are recombined under the effect of Marangoni convection induced by the interfacial tension gradient²⁷. Ti preferentially combines with Si to form compounds, with the first precipitated Ti₅Si₃ serving as heterogeneous nucleation points in the subsequent nucleation process²⁸. The remaining active elements react to generate carbides such as TiC and Ti₈C₅ and intermetallic compounds like Ti₃Al. Due to the relatively simple combination of Ti and C element, the reaction of intermetallic elements will be slightly delayed. As the temperature decreases, the material will further solidify, aggregation phenomenon will occur between some of the phases to form dendritic crystals, while in other regions of the molten pool, ellipsoidal and spherical equiaxial crystals will form later.

Fig. 8

Schematic diagram of the melt pool evolution process.

Microhardness

Figure 9 illustrates the microhardness of the cross-section of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings and the variation of hardness along the depth direction of the coatings. Figure 9a shows that the average microhardness of Cr₀, Cr₅, Cr₁₀ and Cr₁₅ coatings are 841.0HV_0.2, 903.3HV_0.2, 926.0HV_0.2 and 989.5HV_0.2, which are 2.3, 2.4, 2.5, 2.7 times higher than that of TC4 (370.8HV_0.2), respectively. A monotonic increase in average microhardness was observed with increasing Cr content. This is due to the precipitation of hard phases, such as TiC, Ti₈C₅, Ti₅Si₃ and Cr₂AlC in the process of laser cladding. The gradual increase of hard phases and grain refinement result in a gradual increase in the microhardness of the coatings.

Fig. 9

The hardness of Ti-Al-Si-SiC-Cr_x (X = 0, 5, 10, 15). (a) Average hardness of coating surface. (b) Cross section hardness curves.

Figure 9b shows the trend of microhardness of the coating along the depth of the laser cladding layer. The microhardness of the cladding layer is significantly higher than that of the substrate and decreases gradually in the heat-affected zone. This is due to the fact that the closer to the bottom of the melt pool, the smaller the thermal influence of laser cladding, the larger the grain size, and the lower the proportion of hard phase, resulting in a decrease in microhardness. In addition, the hardness in the heat-affected zone is still higher than that of the TC4 substrate due to the phase hardening and grain refinement of the material in the heat-affected zone. Data analysis confirms that variations in average values are consistent with theoretical predictions of Cr-induced microstructural changes. Given the monotonic increase in hardness and inter-group differences significantly exceeding typical instrument measurement errors, these variations are statistically significant and practically significant.

Indentation hardness and Young’s modulus

Figure 10a illustrates the load-displacement curves for nanoindentation experiment of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings. During indentation testing, owing to the difficulty in isolating individual phases, a grid indentation array (spacing: 2 mm) was employed to determine the indentation locations. Seven indentations were performed on each coating and the results were averaged to obtain the overall mechanical properties of the region. The curves show that the load-displacement curves are continuous and smooth from the time the indenter is pressed into the coating to the end of the indentation experiments. Notably, no significant step features are observed within these curves, which indicates that the coatings are free from cracking and crushing during the loading process²⁹.

The maximum load, indentation hardness and Young’s modulus of the four coatings are given in Fig. 10b. At an indentation depth of 1000 nm, the maximum load required for the four coatings increased by 47, 61, 86 and 129 mN and the indentation hardness increased by 2.387, 3.758, 6.319 and 7.599 GPa, respectively, relative to TC4 (88 mN, 4.174 GPa). It can be seen that with the increase of Cr content, Cr₂AlC hard phase was precipitated in the coating, and the proportion of Ti₅Si₃ hard phase increased, which led to a gradual increase in the indentation hardness of the coating.

Fig. 10

Nanoindentation load-displacement curve of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coating. (a) Load displacement curve. (b) Comparison diagram of maximum load, hardness and Young’s modulus.

The Young’s modulus of the four coatings are 167.445, 164.377, 194.231, and 231.972 GPa, respectively, which are also substantially improved compared with that of TC4 (135.101 GPa). The difference in Young’s modulus between Cr₀ and Cr₅ coatings is only 3.068 GPa, indicating that the intermetallic compounds Al₂Cr and Ti₄Cr exert minimal influence on the Young’s modulus.

Dynamic compressive mechanical properties

To investigate the effect of Cr content on the dynamic compression mechanical properties of composite coatings, dynamic compression experiments were performed on TC4 titanium alloy and Ti-Al-SiC-Cr_x composite coatings using a split Hopkinson pressure bar (SHPB) device with a bar diameter of 14.5 mm at room temperature and atmospheric pressure. The experimental setup and testing principle refer to reference³⁰. The bar system utilized steel bars fabricated from 55CrSi spring steel, with an elastic modulus (E) of 210 GPa, a density of 7.85 g/cm³, and an approximate stress wave velocity of 5100 m/s. The striker bar measured 300 mm in length, while both the incident and transmission bars were 1000 mm long. The absorber bar length was 600 mm. During testing, the signal gain for both the incident and transmission bars in the ultra-high dynamic strain acquisition system was set to 500.

Cylindrical specimens are conventionally employed in Split Hopkinson Pressure Bar (SHPB) experiments. To ensure the validity of axial true strain measurements even under significant plastic deformation, the specimen diameter is typically maintained within 80% of the bar diameter (Ø 14.5 mm). Accordingly, specimens with a diameter of 5 mm and a thickness (length) of 4 mm were designed based on the existing bar diameter. To ensure data reliability, each experiment was repeated in triplicate.

Figure 11 illustrates the stress-strain curve derived from dynamic compression tests on laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings. Subfigures (a), (b), (c) and (d) illustrate the stress-strain responses for Cr₀, Cr₅, Cr₁₀, and Cr₁₅ coatings, respectively. As observed in the figure, the peak stress of the coating increases with the increase of strain rate, demonstrating a distinct strain rate strengthening effect.

Fig. 11

Dynamic compressive stress-strain curves of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coating. (a) Ti-Al-SiC-Cr₀. (b) Ti-Al-SiC-Cr₅. (c) Ti-Al-SiC-Cr₁₀. (d) Ti-Al-SiC-Cr₁₅.

The yield strengths of the Cr₁₅ coating at 300 s⁻¹, as well as the peak stresses and peak strains of all four coatings at 120 s⁻¹, 180 s⁻¹, and 300 s⁻¹, have been extracted from Fig. 11 and are presented in Fig. 12. Since no distinct yielding phase is observed in the stress-strain curves, the stress value corresponding to 0.2% plastic strain is taken as the yield strength.

Fig. 12

Comparison of dynamic compressive mechanical property parameters of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15). (a) Yield strength. (b) Peak stress and strain.

As shown in Fig. 12a, the yield strength of the four coatings increases from 720 MPa for Cr₀ to 945 MPa for Cr₁₅ at a strain rate of 300 s⁻¹. The yield strength of the coating exhibits a gradual increase with rising Cr content. As illustrated in Fig. 12b, the peak stresses of the four coatings are similar at each loading strain rate. The peak strains of four coatings at identical peak stresses decrease gradually with increasing Cr content. For example, at a strain rate of 300 s⁻¹, the strains of the four coatings are 0.0313, 0.298, 0.276 and 0.261, respectively, indicating that the strength and plastic deformation resistance of the coatings increase with increasing Cr content under high strain rate loading. The above phenomenon is attributed to the precipitation of the Cr₂AlC phase and the increase of the Ti₅Si₃ phase content as the Cr content increases, which leads to an increase in the hardness of the coating. On the other hand, grain refinement leads to an increase in the total area of grain boundaries and an increase in dislocation barriers. This means that an increase in the number of grains with different orientations needs to be coordinated, thereby giving the coating greater plastic deformation resistance.

Wear resistance

To evaluate the tribological properties of the laser cladding Ti-Al-SiC-Cr_x (x = 0, 5, 10, 15) coatings, sliding wear tests were conducted at room temperature. The tests were performed under a load of 10 N, a reciprocating frequency of 5 Hz, a wear track diameter of 6 mm, and a duration of 30 min. All tests were repeated three times, and the average values were reported as the final results. The friction curves and schematic diagram of the coatings are presented in Fig. 13a-d. During the initial stage of the friction experiments, the relative friction between the friction pair and the coating is unstable, resulting in some fluctuations in the initial stage of the curves³¹. The coefficient of friction (COF) curves of different coatings exhibit a typical sawtooth fluctuation patter due to the periodic accumulation and elimination of wear debris on the worn surface³². As illustrated in Fig. 13a, the friction coefficients of the four coatings are 0.43, 0.46, 0.5, and 0.56 respectively, and the results indicate that the friction coefficients of the coatings increase gradually with the increase of Cr content. This phenomenon can be attributed to the fact that as Cr content increases, the presence of Cr₂AlC and Ti₅Si₃ hard phases within the coating increases, subsequently enhancing the surface hardness of the coating. In contrast, for TC4 and coatings with a lower Cr content, the soft phases within the coating act as lubricants during the friction and wear process, thereby reducing the coefficient of friction. On the other hand, the contact state and contact area of the friction pair also have an effect on the friction coefficient. As the hardness of the coating increases, fragments of the hard phase are prone to adhering to the friction pair, which leads to an increase in the contact area between the friction pair and the coating, thereby resulting in an increase of coating friction coefficient^33,34.

Fig. 13

Friction curves of Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings at room temperature. (a) Friction curve. (b) Wear volume and wear rate. (c) Scar depth-width curve. (d) Schematic of frictional wear.

Figure 13b compares the wear volume and wear rate of the four coatings. The wear loss of all four coatings is lower than that of TC4 (1.73 mg), with the Cr₅ coating exhibiting the lowest wear loss of 1.44 mg. The wear rate of the coating is calculated based on the wear volume using Archard’s wear rate equation W = V/(L×S), where V is the wear volume in mm³, L is the normal force in N, and S is the total sliding distance in m³⁵. The wear rates of the four laser cladding coatings are as follows: 2.25 mm³ N⁻¹m⁻¹ for Ti-Al-SiC-Cr₀, 0.75 mm³ N⁻¹m⁻¹ for Ti-Al-SiC-Cr₅, 1.75 mm³ N⁻¹m⁻¹ for Ti-Al-SiC-Cr₁₀, and 1.37 mm³ N⁻¹m⁻¹ for Ti-Al-SiC-Cr₁₅. In general, material hardness positively correlates with wear resistance. However, experimental results indicate that the wear amount and rate of Cr₁₀ and Cr₁₅ coatings are greater than those of the Cr₅ coating. This anomaly can be attributed to the increased presence of hard phases in the Cr₁₀ and Cr₁₅ coatings. During the wear process, generated wear debris tends to adhere to the coating surface, resulting in friction between the wear debris and the friction pair, which leads to an increase in the wear amount and wear rate.

Figure 13c shows the depth-width curves of the friction scars, as shown in the Ti-Al-SiC-Cr_x coatings, the widths of the wear scars of the Cr₀, Cr₅, Cr₁₀, and Cr₁₅ coatings are 1657 μm, 1253 μm, 1558 μm, and 1465 μm. The Cr₅ coating presents the smallest depth and width of the wear scars. The Cr₁₀ and Cr₁₅ coatings exhibit similar wear depth and width, while the Cr₀ coating exhibits the largest wear depth and width. The results show that the Cr₅ coating have better wear resistance while the Cr₀ coating have poorer wear resistance. In the Cr₅ coating, Ti₃Al (a soft phase) and hard particles (Ti₄Cr, TiC, and Ti₅Si₃) co-form an optimized “soft-hard composite” structure. During the friction process, the soft Ti₃Al phase undergoes selective plastic deformation, forming a continuous, dense lubricating film on the worn surface. Simultaneously, finely dispersed spherical TiC and Ti₅Si₃ particles resist abrasive wear plowing. EBSD analysis revealed a larger average grain size (3.3 μm) and higher proportion of HAGBs (92.37%) in the Cr₅ coating. This microstructure deflects and blunts microcracks, inhibits propagation along brittle phases, and suppresses macroscopic brittle delamination.

Figure 14a-d illustrates the 3D morphology of wear scars for laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings. The wear area is denoted by the blue region, and the darker the color, the more severe the wear.

Fig. 14

The 3D morphology of wear scars of laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings. (a) Ti-Al-SiC-Cr₀. (b) Ti-Al-SiC-Cr₅. (c) Ti-Al-SiC-Cr₁₀. (d) Ti-Al-SiC-Cr₁₅.

As illustrated in the figure, the wear scars in the Cr₁₀ and Cr₁₅ coatings exhibit a fluctuating groove pattern. This can be attributed to the presence of harder phases such as TiC, Ti₈C₅, Ti₅Si₃, and Cr₂AlC in the wear debris of these coatings during the wear process. The wear debris predominantly exhibits a granular morphology, accompanied by a small amount of clumps. The hard phases are torn and agglomerated on the coating surface under the action of the Si₃N₄ ceramic balls, forming granular debris that undergoes abrasive wear with the Si₃N₄ ceramic balls and the coating surface. However, the wear scars of the Cr₅ coating show slight grooves. This is due to the existence of Al₂Cr, Ti₃Al and Ti₄Cr soft phases in the Cr₅ coating, which produce flake wear debris for lubrication. Furthermore, The Cr₅ coating exhibits a lower hard phase content compared to the Cr₁₀ and Cr₁₅ coatings, resulting in relatively less granular debris.

Conclusion

• 1. The laser cladding Ti-Al-SiC-Cr_x (X = 0, 5, 10, 15) coatings primarily consist of TiC, Ti₃Al, Ti₈C₅, and Ti₅Si₃ phases. As the Cr content increases, the Ti₄Cr and Cr₂AlC phases gradually precipitate, accompanied by a reduction in the proportion of the ellipsoidal Ti₈C₅ phase and an increase in the long block-like Ti₅Si₃ phase within the microstructure. Concurrently, the grain size is refined, and the fraction of low-angle grain boundaries progressively rises.

• 2. The surface microhardness of the coatings enhances with increasing Cr content, although it diminishes gradually from the heat-affected zone toward the substrate. During nanoindentation experiments, the coatings exhibit no signs of cracking or crushing. Furthermore, the maximum load, indentation hardness, and Young’s modulus of the coatings consistently improve with higher Cr content.

• 3. Under dynamic compressive loading, the stresses of the cladding coatings increase with increasing strain. Both the yield strength and peak stress of the coatings rise with higher strain rates, demonstrating a pronounced strain rate strengthening effect. At a constant strain rate, the yield strength of the coatings increases, while the peak strain decreases as the Cr content increases.

• 4. During the friction and wear process, the soft Ti₄Cr phase acts as a lubricant, while the hard Ti₅Si₃ and Cr₂AlC phases, which increase in proportion with higher Cr content, contribute to an elevated friction coefficient. Among the coatings, Cr₅ exhibits the lowest wear rate. Compared to Cr₀, Cr₁₀, and Cr₁₅ coatings, Cr₅ demonstrates superior plasticity while maintaining high strength and hardness, showcasing the best overall performance.