Effect of CeO₂ on TiC Morphology in Ni-Based Composite Coating

Introduction

Ceramic metal coating, using laser cladding technology, can greatly improve the performance of the material surface, because that the composite coating has excellent hardness, thermal stability, corrosion resistance, wear resistance, and metallurgical bonding with the matrix [1]. The thermal expansion coefficient, elastic modulus, and coefficient of thermal conductivity between the ceramic and metal matrix are varied greatly due to the melting point of ceramic phase is much higher than the substrate. As a result, the temperature gradient in the molten pool is great under the condition of the same laser irradiation. The defects, such as holes and micro-cracks, existed in the cladding layer via the thermal stress caused by the temperature gradient [2].

Rare earth, as one of the surface active elements, can decrease the surface tension. The amount of the core and crystallisation rate can be increased by reducing the work of nucleation, at last, the microstructure of the cladding layer be refined [3]. Rare earth is easy to react with oxygen, sulphur, silicon, and other elements to generate the stable, low melting point compounds; the microstructure will be purified because the compounds will rise from the liquid phase and form the slag on the surface of the cladding layer. A result shows that the influence of rare earth on the microstructure and mechanical properties of the laser cladding layer is one of the hotspots in laser cladding technology. Kunlin Wang et al. researched the effect of La₂O₃ on the microstructure and wear resistance of the nickel-based alloy laser cladding layer [4]. Results showed that La₂O₃ could refine and purify the microstructure, reduce the dilution rate of the cladding layer and the lattice constant of solid solution, increase the micro-hardness and decrease the friction coefficient, the wear resistance was improved. Y. M. Zhang et al. analysed the microstructure of 4wt% CeO₂+20wt% WC+ NiCrBSi laser cladding layer via X-ray diffraction (XRD) and transmission electron microscopy (SEM) [5]. The result found that the grain size of the γ-(Fe, Ni) phase before and after adding rare earth in the coating was 3.0 μm and 1.5 μm, respectively, thus, the grain of organisation was refined significantly. The new phase CeNi₈Si₅ was formed in the composite coating after adding rare earth, and the content of some carbide, such as (CrW)₇C₃, was enhanced.

To sum up, scholars systematically investigated the effect of rare-earth elements on the metal matrix in the cladding layer, but the effect of rare earth on ceramic metal coating is rarely studied. The influence mechanism of rare earth on the microstructure, phase composition, and properties of the ceramic metal coating is not clear.

Consequently, the CeO₂ was selected to add in the TiC/Ni composite powder, cladding a ceramic-metal layer on the surface of Cr12MoV steel. The effect of different content of rare earth on the microstructure, micro-hardness and wear resistance of the TiC/Ni composite coating was studied systematically.

Experimental procedures

Cr12MoV steel scaled 300×200×10 mm in bulk was used as a work piece in the study. The steel was conducted by general annealing (i. e. 850~870°C×3~4 h, cooled to 740~760°C×4~5 h, air cooling) before the test. The nominal chemical composition of Cr12MoV steel is shown in Table 1. The chemical composition of cladding layer powders is shown in Table 2; the ratio of TiC and Ni60 was 1/9, and the micro-hardness of TiC was 3000 kg/mm². The nominal chemical composition of Ni-based alloy powders is shown in Table 3. The purity of the powder was higher than 99.5 wt%, and the particle size was less than 60 μm. The average particle sizes ranging was 30–40 μm after the mixing operation.

Figure 1 shows the schematic diagram of the laser cladding. Substrates were grounded with sand paper and then cleaned with alcohol to remove any organic elements. Then, the cladding powders were mixed with alcohol and the concentration of the powders was similar to the cream. Finally, the powders were coated on the surface of the substrate, and the thickness was about 1.2 mm. The JK2003SM type Nd: YAG laser equipment was used to clad the powders on the substrate, the welding parameters are shown in Table 4, 1# parameters were used simple tracks laser, and 2# parameters were used the multi-tracks laser with overlap.

Figure 1:

Schematic of laser cladding process.

After the laser cladding, the cross-section of the cladding layer was studied by the Olympus stereomicroscope, and the microstructure of the composite coating was characterised by the Olympus optical microscope, S4800 scanning electron microscopy (SEM) and JEM-2100F transmission electron microscope (TEM). The phase of the polished coating was identified by a D8 Advance X-ray diffractometer (XRD, 40 kV, 40 mA, Cu Kα radiation, scanning speed was 4°/min), and the OrignPro 8.5 software was used to analyse the data. An Image-Pro Plus image analyser was used to determine the shape and size of TiC within the composite coating and the average size of primary crystallisation. The micro-hardness distribution from the surface of the composite coating to the substrate was measured by an MHV2000 type digital micro-hardness tester with a 100 g load, and 10 s dwell time; the distance between the two points was 0.2 mm.

An MM-200 dry sliding wear tester was used to conduct the wear resistance test. The ring material of the friction pairs was GCr15 with macro-hardness 60 HRC. The outer radius of the wear ring was 50 mm, and the width was 10 mm. The size of test specimens was 25×7×7 mm. The wear load is 49N, the sliding speed is 200r/min, and the sliding time is 120min. Equation (1) was used to calculate the wear volume:

where B is the width of specimens (mm), L is the width of wear track (mm), and R is the outer radius of wear ring (mm). The weight of specimens before and after the test was measured by an analytical balance, and then the weight loss was calculated. After the wear test, the worn surface of the composite coating was studied by an S4800 scanning electron microscopy (SEM).

Results and discussion

Macro-morphology of the cladding layer

Figures 2 and 3 show the surface morphology and cross-section of cladding layers with different content of CeO₂ respectively. The surface of composite coatings is smooth and continuous without obvious defects, and the metallurgical bonding between coating and substrate is excellent. The width of the dilution zone, clad height and width are about 0.4 mm, 1 mm and 2 mm respectively. The surface contour lines of cladding layers are all convex curve.

Figure 2:

Macro-morphology of the composite coatings: (a) 0 %; (b) 2 %; (c) 4 % and (d) 6 %.

Figure 3:

Cross-section of the composite coatings: (a) 0 %; (b) 2 %; (c) 4 % and (d) 6 %.

The surface tension has a close relation with the temperature [6], and the eq. (2) shows the relationship.

(2)σ=σ0−ST

where T is the temperature of the system, σ₀ is surface enthalpy, S is surface entropy, σ is surface tension. When the system is under constant pressure conditions, eq. (2) translates to eq. (3), as follows.

(3)∂σ∂TP=−S+T∂S∂TP

The value of ∂σ∂TP is constantly less than zero because of S>0 and T∂S∂TP>0, namely the surface tension will be decreased with the increase of the temperature. The temperature of the internal cladding layer metal was higher than that of external for the centre of the laser spot has higher energy compared with outside [7]. Hence, the surface tension of the cladding layer will be increased from the centre to the outside. According to the principle of minimum free energy, the outside cladding layer metal will be gathered to the centre during the laser cladding. Accordingly, all of the cladding layers have the similar convex contour line.

Microstructure of the cladding layer

The microstructure of composite coatings is all composed of solid solutions and hard particles via analysing the XRD patterns of composite coatings before wear test, as shown in Figure 4. The phases of the cladding layer, without rare earth, are γ-(Fe, Ni), TiC and a spot of (Cr,Fe)₇C₃ and Fe₃C. The hard particles, including the original ceramic particles and the new particles, distribute on the soft matrix metal could improve the wear resistance of the cladding layers. Some new phases, such as Ce₃Ni₆Si₂ and CeNi₃, occurred in the cladding layer when adding different content of CeO₂. The d of some peaks (i. e. CeNi₃, Ce₃Ni₆Si₂ and TiC) in the four kinds of samples is list in Table 5. The content of rare-earth compounds (i. e. CeNi₃, Ce₃Ni₆Si₂) is increasing with the increase of cerium oxide content. However, the content of TiC has the opposite change law compared to the rare-earth compounds.

Figure 4:

XRD patterns of the composite coatings before wear test.

Table 5:

d of peaks in the four kinds of samples.

Phase	CeNi3	Ce3Ni6Si2	TiC
A1 Coating	–	–	0.23526
			0.21587
			0.16275
A2 Coating	0.24648	0.19715	0.23501
	0.18607	0.19261	0.21495
	0.12364	0.12528	0.15869
A3 Coating	0.26192	0.19740	0.23491
	0.18549	0.19261	0.21466
	0.12367	0.12551	0.15853
A4 Coating	0.26225	0.19790	0.23419
	0.18607	0.19261	0.21456
	0.12394	0.12583	0.15854

Evolution law of the cladding layers microstructure

Figure 5 shows the microstructure of cladding layers with different content of CeO₂. The microstructure of cladding layers has the same evolution law, namely the microstructure from the bottom to the surface is columnar crystal, cellular crystal, and equiaxed crystal respectively. The phenomenon can be explained by the one-dimensional heat conduction model [8].

Figure 5:

SEM microstructure of the cladding layers: (a) 0 %; (b) 2 %; (c) 4 % and (d) 6 %.

In the initial stage of molten pool solidification, the temperature gradient between the bottom of the molten pool and matrix is great, and solidification speed (R) tends to zero, so the value of G/R tends to infinity. The coarse columnar crystal will grow along the normal direction of the substrate because the heat dissipation speed at the direction of perpendicular to the substrate is the highest. After the formation of the columnar crystal, the cooling rate dropped significantly for the increase of substrate temperature. The G/R value decreased for the temperature gradient descent and solidification rate (R) increases. So, the bulky cellular crystal will occur at the stage. At the late stage of molten pool solidification, the heat dissipation speed will be reduced with the increase of solid body thickness. The value of G/R becomes smaller for the temperature gradient decreases and solidification speed increases. Furthermore, there exist a large amount of slag and impurities on the surface of the molten pool that makes the increase of nucleation site. As a result, the microstructure in the top of the cladding layer is fine equiaxed crystal.

Table 6 shows the average size of primary crystallisation in the four kinds of samples. The microstructure of the same position in different cladding layers is compared to find that, there exists coarse and nonuniform microstructure without CeO₂, as shown in Figure 5(a). The microstructure will become uniform and tiny with the increase of CeO₂ content. And the cladding layer achieves the excellent microstructure when the content of CeO₂ is 4 %, as shown in Figure 5(c). While the CeO₂ content more than the optimal value, the microstructure becomes coarse and nonuniform again, as shown in Figure 5(d). The following two aspects can be used to interpret the above experimental phenomenon:

Table 6:

Average size of primary crystallisation (μm).

Group	Columnar crystal		Cellular crystal	Equiaxed crystal
	Length	Width
A1 Coating	27.3	7.3	15.3	12.7
A2 Coating	20.9	6.1	9.5	7.3
A3 Coating	19.7	5.5	6.1	4.5
A4 Coating	22.7	6.4	11.1	9.1

(1) The constitutional supercooling tendency in the solidification process will be increased because of rare-earth elements mainly gather on the liquid side of the liquid–solid interface. The formation speed and amount of dendrite crystal at the columnar crystal will be increased. At the same time, rare-earth elements can reduce the melting point of the dendrite crystal that makes the dendrite crystal fusing; the dissociative dendrite crystal could be as the new crystal nucleus that refines the grain in the cladding layer [9]. The interfacial tension was reduced because of the rare-earth elements segregate at the boundaries of dendritic crystal after crystallisation [10]. As a result, the driving force of grain growth is reduced that refine the microstructure of the cladding layer effectively. When the content of the rare-earth elements is more than the optimal value, the rare earth loses the restriction to the grain growth because of grain boundaries are polluted. The microstructure of the cladding layer will become bulky again.

(2) Rare-earth elements can form inter-metallic compounds with other elements in the cladding layer. The tiny inter-metallic compounds can be used as nucleation cores to reduce the nucleation work and improve the nucleation rate, so as to refine the microstructure.

Evolution law of TiC ceramic particles

In the process of rapid laser heating, the solubility of TiC in the molten pool is bigger than Ni-based alloy powders because the TiC powder has higher absorption of laser energy. Therefore, some tiny TiC particles were completely dissolved, and the edge and convex of larger TiC particles also appeared partly dissolved [11]. The sharp corner position in the TiC particles has a higher surface free energy that makes the rare earth segregate on its surface [12], as shown in Figure 6. The rare-earth elements will accelerate the fusion and decomposition of TiC surface because the rare-earth elements could reduce the melting point of the alloy system.

Figure 6:

Distribution of elements in the microstructure of A3 cladding layer.

A part of the Ti element and C element in the dissolved TiC ceramic particles will generate new TiC precipitate from γ-(Fe, Ni), as shown in Figure 7. The grain refinement of the rare-earth element makes the precipitated TiC particles have the less size. The new TiC particles have good metallurgy combination with alloy substrate. Figure 6 shows that the Ti element only distributes on the TiC position and its surrounding. Thus, the balance Ti element will generate some Ti-rich compounds with other alloy elements around the original TiC particles, which improved the wettability between the original ceramic particles and metal matrix [13].

Figure 7:

(a) TEM micrograph of matrix with reinforce particle; (b) SAED pattern of reinforce particle; (c) SAED pattern of matrix.

The appropriate amount of rare-earth elements not only accelerated the dissolution of the original ceramic particles but also optimised the shape and size of the original and new ceramic particles [14].

The shape factor (Δ) of TiC can be calculated by the eq. (5) [15].

(5)Δ=ab

In eq. (5), Δ is the shape factor of TiC, a and b are the maximum length and the minimum width of TiC, respectively.

Table 7 shows the shape factor and size of TiC (i. e. undissolved and new TiC particles). The TiC particles have the larger size and irregular shape without rare earth. The size of TiC particles decrease with the increase of rare earth content, and the shape of particles approach to sphericity. On the one hand, the rare-earth element could accelerate the fusion and decomposition of the sharp corner position in the original TiC particles. On the other hand, the rare-earth element could spherize and refines the new TiC particles that precipitate from the matrix. The content of TiC particles in the four cladding layers is about 9.1 %, 7.9 %, 6.4 % and 3.1 % respectively. The content of TiC particles in the A4 coating is obviously less than other three coatings because the higher rare-earth content will lead to excess fusion of the original TiC particles.

Table 7:

Shape factor and size of TiC.

Group	Δ (%)					Average size (μm)
	1～1.2	1.2～2	2～3	3～5	＞5
A1 Coating	24.5	14.5	28	25.5	7.5	4.0
A2 Coating	40.5	26.0	11.5	16.5	6.5	2.2
A3 Coating	54.5	23.5	14.0	3.5	2.5	1.4
A4 Coating	65.5	20.5	11.5	2.5	0	1.0

Wear performance

Figure 8 shows the micro-hardness distribution curve from the surface of the cladding layer to the substrate with different CeO₂ content. When the content of CeO₂ is 0 %, the cladding layer has lower micro-hardness. The micro-hardness will be increased with the increase of CeO₂ content, and the micro-hardness achieves the maximum when the content of CeO₂ is 4 %. Compared with 4 % content, the micro-hardness decreased when the content of CeO₂ is 6 %. Furthermore, the microhardness of the heat affected zone (i. e. 720 MPa-930 MPa) is higher than that of the composite coating (i. e. 220 MPa). Because tempered martensite and network carbide existed in the heat affected zone that increased the microhardness, as shown in Figure 9.

Figure 8:

Micro-hardness distribution of the composite coating.

Figure 9:

Microstructure of the heat affected zone.

Analysing the wear resistance data of the cladding layer with the different content of rare earth, as shown in Figure 10, find that. The cladding layer adding with rare earth has higher wear resistance than that of without rare earth. The cladding layer with 4 % CeO₂ has the optimum wear resistance. The wear resistance will be decreased when the CeO₂ content less or higher than the optimal value.

Figure 10:

Wear losses of the composite coating.

Figure 11 is the worn surface morphologies of the composite coating with the different CeO₂ content. The result shows that, when the content of CeO₂ is 0 %, there exist many deep and wide ploughing grooves (PG) and couple with some smooth region (SR) and shedding phenomenon (①) on the worn surface, as shown in Figure 11(a). When the CeO₂ content is 2 %, the ploughing grooves on the worn surface become shallow and narrow, and the shedding phenomenon disappeared. When the CeO₂ content is 4 %, the shallow and narrow ploughing grooves uniformly distribute on the worn surface. When the CeO₂ content is 6 %, the ploughing grooves on the worn surface become deep and wide, and there exist large-area plastic deformation.

Figure 11:

Worn surface morphologies of the composite coating: (a) 0 %; (b) 2 %; (c) 4 % and (d) 6 %.

Based on the above experimental data find that the wear resistance of the cladding layer has a close relation with the microstructure and the shape, size and distribution of TiC ceramic particles.

The cladding layer has coarse and nonuniform microstructure when the content of CeO₂ is 0 %. TiC ceramic particles have the larger size for it easy to reunite in the laser cladding process. The wettability between hard phase and matrix alloy will be reduced that make the metallurgical bonding force decrease. Therefore, the plough in the microstructure of the worn surface is a deep and wide couple with some shedding phenomenon, as shown in Figure 11(a). The cladding layer has the lowest wear resistance among the four specimens.

After adding appropriate CeO₂ (4 %), some rare-earth compounds (Ce₃Ni₆Si₂, CeNi₃) can be formed in the laser cladding process that has lubrication during the wear test. The formation and precipitation of carbides also can be promoted; the plough and adhesion effect will be reduced via improving the hardness of the cladding layer. A large amount of tiny TiC particles precipitate in the alloy matrix and dispersively distribute in the cladding layer, the metallurgical combination force between precipitated particles and metal substrate will be increased. Besides, the unfused TiC ceramic particles will be spheroidised and refined. As a result, the wear resistance of the cladding layer is improved significantly; the plough in the worn surface is shallow and narrow without the shedding phenomenon occurs, as shown in Figure 11(c).

As the content of CeO₂ is 6 %, the microstructure of the cladding layer will grow up again and become coarse and nonuniform for the pollution of grain boundary. The excessive rare-earth elements will aggravate the decomposition of TiC particles. As a result, the hardness and the wear resistance of the cladding layer will be decreased because of a large amount of TiC particles are burned loss. Therefore, the cladding layer will occur severe plastic deformation during the wear test, and the plough is deep and wide, as shown in Figure 11(d).

Conclusions

1. The TiC/Ni composite coating was successfully fabricated by laser cladding. The metallurgical combination between the composite coating and the substrate is good.

2. The main phases in the cladding layer are γ-(Fe,Ni), TiC, (Cr,Fe)₇C₃ and Fe₃C without rare earth. After adding CeO₂, some new phases occur in the cladding layer, such as Ce₃Ni₆Si₂ and CeNi₃.

3. The microstructure of the cladding layer with the different content of CeO₂ from the bottom to the surface is columnar crystal, cellular crystal and equiaxed crystal.

4. The cladding layer has coarse and nonuniform microstructure when the content of CeO₂ is 0 %, and the wear resistance is lower. Appropriate rare-earth elements could refine and homogenise the microstructure and enhance the content of carbides, and rare-earth compounds have the lubrication in the wear test. Consequently, the wear resistance will be improved significantly. The excessive rare-earth elements can pollute the grain boundaries; the wear resistance will be decreased for the microstructure of the cladding layer become coarse and nonuniform.

5. TiC particles will gather in the cladding layer without rare-earth elements that will decrease the metallurgical combination with the matrix, and then the wear resistance will be reduced. Appropriate rare-earth element could spheroidise and refine the precipitated TiC particles, and original TiC particles, the metallurgical combination of ceramic reinforced phase and matrix will be increased. Therefore, the wear resistance of the cladding layer was enhanced significantly. The excessive rare-earth elements can make the excessive burning loss of TiC particles that reduced the wear resistance of the cladding layer.