Microstructure and Tribological Performance of TiB₂-NiCr Composite Coating Deposited by APS

Abstract

Nickel chromium (NiCr) powders with different titanium diboride (TiB₂) additions (20, 40 and 60 wt %) were prepared with a mechanical alloying method and then sprayed using an air plasma spraying technology. The microstructure and phase composite of the powders and the cross-sections of deposited coatings were analyzed with a scanning electronic microscope and X-ray diffraction. The tribological performance of the coatings was studied using a pin-on-disk tribometer at room temperature. The weight loss of the as-sprayed coating was measured by using a high accuracy weighing balance. Cr₃C₂-25NiCr coating was produced and tested for comparison. The morphologies of the worn surface were then investigated. Parts of debris with some scratches were found, presenting typical signs of abrasive wear and showing slight adhesive wear on the surface. The 20 wt % additive TiB₂ coating demonstrated the highest microhardness and the lowest coefficient of friction. The wear resistance of the metal-ceramic composites coatings was enhanced with the addition of TiB₂.

1. Introduction

Automobile brakes are the most pivotal and demanding component in the automobile safety system [1]. According to statistics, 30% of automobile accidents in the country are caused by brake failure, and poor quality brake pads are the main cause of failure. Semi-metal friction materials, such as steel fibers, are widely used in China as brake pads [2]. A new material is needed since the steel fibers may cause brake shrinkage and brake fluid boiling when the temperature increases. Thermal sprayed cermet coatings on the running surface of the brake pads have been considered as an alternative method to solve this problem [3,4].

As a potential boride ceramic material, pure titanium diboride (TiB₂) has some excellent properties such as high hardness (34 GPa), a high melting point (2980 °C), a higher Young’s modulus (414 GPa at 1090 °C) and a lower friction coefficient compared to Fe [5,6]. Therefore, an increasing number of studies have focused on producing TiB₂-based ceramic coatings [7,8,9,10]. Wang et al. [11] cladded TiB₂ and TiC powders on an AISI 1045 steel surface and found that the wear resistance was improved in dry sliding conditions. Dallaire et al. [12] tried to create flux-cored wires composed of 304 stainless steel shells filled with additive of 10–65 wt % TiB₂ powders, which was then sprayed using the twin arc spraying method. James et al. [13] investigated the composition of Ni and TiB₂ coatings and sputtering operation parameters to protect substrates from solid particle erosion. However, limited research exists on the tribological performance of TiB₂ and NiCr coatings in thermal spraying.

Pure TiB₂ is brittle and easily reacts with oxygen at high temperature, so it should cohere to soft materials to form a metal-ceramic coating, such as NiCr alloy [14,15]. In this work, TiB₂ and NiCr powders with different additions of TiB₂ (20, 40 and 60 wt %) were mixed using mechanical alloying (MA) and then sprayed using air plasma spraying (APS) technology. The effect of adding TiB₂ on the tribological performance of the coating was then investigated.

2. Experimental Procedure

The source material for the surface coating was a mixture of the Ni₈₀Cr₂₀ alloy powder (Ti Metal Materials Co., Ltd., Changsha, China) and TiB₂ powder (Xiangtian Nano Materials Co., Ltd., Shanghai, China) selected using a mesh size of 300, with the ratios of TiB₂ mass fraction of 20%, 40% and 60%. The two powders were mixed using the ball-milling method. The 304 stainless steel balls (ball-to-powder weight ratio of 15:1) were put into a planetary ball grinding mill (KQM-X/B, Xianyang Jinhong General Machinery Co., Ltd., Xianyang, China) and milled with a rotation speed of 300 r/min for 20 h. The powders were classified into three types: 20TiB₂-NiCr, 40TiB₂-NiCr and 60TiB₂-NiCr. Another commercial powder labeled Cr₃C₂-25NiCr (Praxair Co., Ltd., Danbury, CT, USA) was selected to prepare the coatings for comparison. The compositions of TiB₂, NiCr and Cr₃C₂-25NiCr powders are listed in

Table 1. Compositions of the powders (wt %).

Materials	Ti	B	Fe	Co	Mn	C	N	O	Cr	Si	Ni
TiB2	Bal.	30.75	0.14	0.011	0.03	0.13	0.2	0.5	–	–	–
NiCr	–	–	–	–	–	0.01	–	0.025	19.86	0.02	Bal
Cr3C2-25NiCr	–	–	–	–	–	10	–	–	Bal.	0	20

.

These powders were deposited on Q235 steel substrate (80 mm × 30 mm × 30 mm) by using the SG-100DC plasma torch (Praxair 3710, Praxair, Cleveland, OH, USA). The plasma spray parameters are shown in

Table 2. Plasma spraying operation parameters.

Parameters	Value
Current (A)	600
Voltage (V)	40
Ar/H2 (L/min)	40/10
Powder feed rate (g/min)	30–40
Gun traverse speed (mm/s)	100
Spraying distance (mm)	100
Pre-heating temperature (°C)	150

. The plasma flux was obtained by a mixture of argon (Ar) and helium (H₂) to melt a mass of particles using the relatively high temperature inside the anode. The current intensity was maintained at 600 A, and the H₂ flow rate was 10 L/min, which supplied the heat. Before plasma spraying, these substrate surfaces were sand blasted using Al₂O₃ sand with a mesh size of 30, followed by de-greasing using acetone. When the coatings were sprayed, the samples were cut into small cross-sections for metallography.

The morphologies of the powder and cross-sections samples were observed using a scanning electron microscope (SEM, S-3400N, Hitachi, Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS). In addition, the phase composition of the powders and coatings was characterized using X-ray diffraction (XRD, Shimadzu 7000, Shimadzu, Kyoto, Japan) using monochromatic Cu-Kα radiation (λ = 0.1541 nm) at 40 kV, 30 mA. The Vickers microhardness tester (HVS-1000, TMTeck Instrument Co., Ltd., Beijing, China) was used to measure the coating hardness, with 200g force for 15 s.

To simulate the wear between the friction blocks of the brake pads and the brake disc, gray iron was used as the counterpart [16,17]. The friction tests were performed using an MM-W1 abrasion tester (Jinan Hensgrand Instrument Co., Ltd., Jinan, China) with 100 N loading and 100 r/min during the process.

3. Results and Discussion

3.1. Characterization of Powders

Images of the three different contents of the TiB₂-NiCr powders after ball-milling and Cr₃C₂-25NiCr powders are shown in Figure 1. An enlarged picture is inserted in the top-left for observing the morphology of a single particle. The 20TiB₂-NiCr powder is nearly rounded and it has better liquidity for spraying. The NiCr powders were crushed and adhered to the TiB₂ powder surface after a long ball milling. The TiB₂-NiCr particle is significantly different from the Cr₃C₂-25NiCr particle, which has a hollow structure, as shown in Figure 1d.

Figure 2 shows the XRD analysis results for the three TiB₂-NiCr powders. The Ni and TiB₂ accounted for the majority of the phases of the powders, having a small CrB phase. However, hardly any NiCr phases were detected in the XRD spectrum. During the long time of milling, Ni could react with TiB₂ to form brittle borides and liberate Ti. The mechanical energy was transformed to heat energy, which could provide enough energy for these reactions. These reactions can spontaneously occur through the calculated standard Gibbs free energy [18]. The chemical equation that occurred in the MA is as follows:

TiB₂ = Ti + 2B

(1)

NiCr + B = Ni + CrB

(2)

3.2. Coating Morphology and Phase Composition

The cross-section morphologies of 20TiB₂-NiCr, 40TiB₂-NiCr, 60TiB₂-NiCr and Cr₃C₂-25NiCr coatings are shown in Figure 3. The coatings present a typical and dense lamellar structure [19]. The dark semi-molten or molten TiB₂ particles with different shapes are not uniformly distributed in the lamellas. The reason for this phenomenon is that TiB₂ has a higher melting point, at approximately 3000 °C, and strongly resists plastic deformation even at high temperatures [8]. The ceramic phase in metal ceramic coatings has anti-solid particles or abrasive wear resistance, and the metallic phase protects the ceramic phase from breaking off. Based on the energy spectrum data in

Table 3. The chemical composition (at.%) in different regions.

Figure 3	Zone	Ni	Cr	C	O	Ti	B
(a)	A	0.60	–	–	–	38.78	60.62
	B	26.09	11.54	–	59.28	3.09	–
	C	78.62	19.98	–	–	1.40	–
(c)	E	73.70	12.87	–	–	1.16	12.26
	F	12.97	26.65	–	25.28	25.26	9.86
(d)	G	72.38	14.62	11.11	1.89	–	–
	H	1.23	55.10	43.67	–	–	–

, the three-phase microstructure is clearly seen in Figure 3a. The black-colored circular Zone A is TiB₂, which plays a major role in improving the wear resistance of the materials. The irregular light gray Zone B is mainly Cr₂O₃, which is mainly composed of the oxidation phases in the coating. The remaining grey Zone C is the NiCr bonding phase. However, some gray areas are dissolved Ti and O elements. The deeper the gray scale, the lower the dissolved Ti and O elements.

The distribution of defects is delineated by the large black irregular area, called Zone D (Figure 3b), where the ceramic phase has broken off, and the small black round areas are the pores. In the process of spraying, when these molten particles impact the sprayed coating, a liquid–solid transformation of spray particles occurred due to rapid cooling. The volume shrinkage between the interlayer produced these pores. For these individual particles, the ceramic particles deposited their local heat to the sprayed layer, also forming some closed pores. The microstructure of 60TiB₂-NiCr is similar to the other TiB₂-NiCr coatings, and the chemical compositions in Zones E and F are found in Table 3.

We found that the 20 wt % TiB₂-NiCr coating had a higher deposition rate of about 88%. The most probable reason for this finding is that the additive TiB₂ particles decreased the adherence of the interface [20]. A number of TiB₂ particles hit the substrate and bounced into the air during the plasma spray process, leading to TiB₂ loss. When the mass fraction of TiB₂ reached 60%, the coating was the thinnest at nearly 90 μm, as shown in Figure 3c. Notably, amounts of boride loss can lead to tissue redistribution in cermet coating, called a boride-free path (indicated as λ: the distance from the adjacent boride), which affects the coating mechanical properties.

The two-phase microstructure is clearly shown in Figure 3d, including the light-colored NiCr phase (Zone G) and the gray matrix Cr₃C₂ phase (Zone H). For the NiCr binding phase, different levels of gray represent different levels of dissolved Cr and C elements. The deeper the gray scale, the higher the dissolved Cr and C elements. When the in-flight particles are in the high temperature plasma jet, because the NiCr metal binding phase coated the carbide surface and the binder phase has a lower melting point (1415 °C) compared to carbide (>1500 °C), the binding phase is molten and dissolves near carbide. A similar situation occurs between the NiCr phase and boride. The NiCr binding phase is molten and dissolves near boride.

The XRD analysis results performed on the TiB₂-NiCr coatings are shown in Figure 4. Some new phases occurred, and the Ni phase disappeared compared to the powders. This occurred because, during the spraying process, the Ni first reacted with B as a protective layer to reduce the reaction between O₂ and TiB₂. However, some other phases were still present in the coatings. In Figure 3b, the black belt around the binding phase is Cr₂O₃, with a small amount of TiO₂ and B₂O₃. This is produced by the oxidation reaction between Cr in the binding phase and a small amount of boride with O₂ in the spraying process. Cr mainly occurs during oxidation; Ni and B are generated from the chemical reaction, forming BNi₂ and Ni₄B₃, which are related to the reaction thermodynamics. The oxygen pressure for metal oxide is higher than the decompression pressure of oxide, since the decompression pressure of Cr₂O₃ (1000 °C, 10⁻⁸ MPa) is far lower than the decompression pressure of NiO (1000 °C, 10⁻⁴ MPa), so the Cr₂O₃ phase is mostly generated. However, when the oxidation of boride occurred, hardly any B₂O₃ was produced. During the spraying process, the oxidation and dissolution of boride will decrease in the mechanical properties of the coatings.

Figure 5 shows the microhardness curve of three TiB₂-NiCr coatings and a Cr₃C₂-25NiCr coating from substrate to top coating. The average hardness value of the 20 wt % TiB₂-NiCr coating was 818 HV, with a maximum value of 850 HV. The increase in hardness is attributed to the heterogeneous distribution of TiB₂ particles. The 40 wt % and 60 wt % TiB₂ coatings had average hardness values of about 684 HV and 569 HV, respectively. For the Cr₃C₂-25NiCr coating, the average hardness was about 427 HV, which was lower than all the TiB₂-NiCr coatings. The TiB₂ phase strengthening effect and Cr₃C₂ decomposition during the spraying process were responsible for this effect. The higher levels of NiCr binder phase (20 wt % TiB₂-NiCr) not only increased the coating deposition rate, they maintained TiB₂ particles in this coating to help increase the coating microhardness.

3.3. Tribological Performance of Coatings

Figure 6 shows the friction coefficient curve of the TiB₂-NiCr and Cr₃C₂-25NiCr coatings. The friction coefficient attained a steady state approximately at five minutes of run time. The trend in average friction coefficient as a function of TiB₂ content was mainly similar to that of hardness. The friction coefficient reached a minimum value at 20 wt % TiB₂. The friction coefficient of Cr₃C₂-25NiCr was higher than all the TiB₂-NiCr coatings. TiB₂ is naturally chemically inert, meaning it has a low equilibrium solubility with Fe [21]; hence, finding adhesive wear in a dense TiB₂-phase is difficult.

Figure 7 shows the mass loss of all the coatings after sliding wear testing. The trend in mass loss was similar to that of the friction coefficient. With the addition of the 20 wt % TiB₂, the mass loss was 0.00023 g, which was the best wear resistance. The mass loss of the Cr₃C₂-25NiCr coating was significantly higher than the others, at nearly six-times higher than the 20TiB₂-NiCr coating. At 400 °C, TiB₂-NiCr coatings may form more B₂O₃ phase, which plays an important role in self-lubrication to reduce the mass loss of the surface during the wear process.

By observing the surface after wear, the wear process for these coatings could be analyzed, as shown in Figure 8. Figure 8a shows the worn surface of the 20 wt % TiB₂-NiCr coating, which had the highest hardness. Some debris and some grooves are present, which are typical characteristics of abrasive wear with slight adhesive wear. The hard phase, which mostly included M_xB_y such as TiB₂, TiB, CrB, and Ni₄B₃ from the XRD results, became some new sources of wear. With increasing the additive TiB₂, the grooves gradually disappeared, and some transfer layers appeared. As seen in Figure 8a, the wear process generated a considerable amount of plate-like debris. With the decrease in hardness, abrasive wear will not occur; the wear mechanism changes from abrasive wear to adhesive wear. Compared to the TiB₂-NiCr coating, some small cracks were observed on the Cr₃C₂-25NiCr coating’s worn surface. Once these cracks cause the hard phases to fall off, abrasive particles may be produced that lead to three-body wear. Therefore, the mass loss of the Cr₃C₂-25NiCr coating was the greatest.

4. Conclusions

In this work, TiB₂-NiCr and Cr₃C₂-25NiCr coatings were deposited by using atmosphere plasma spray. Based on the analysis of the microstructure and wear resistance of these coatings, we drew the following conclusions.

• The TiB₂-NiCr coatings had a typical dense lamellar structure. The as-sprayed coating contained mainly TiB₂, Ni₄B₃ and CrB phases.
• The TiB₂-NiCr coatings showed higher hardness, lower mass loss and a lower friction coefficient than the Cr₃C₂-25NiCr coating.
• With the continuous increase in TiB₂ content, the hardness and wear resistance decreased due to the TiB₂ deposition rate. The 20 wt % TiB₂-NiCr coating demonstrated the best wear resistance.
• The wear resistance changed from abrasive wear to adhesive wear when the 40 wt % and 60 wt % TiB₂ were added.