Properties of HVOF-PVD Duplex Coatings: History
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Contributor:
Yingpeng Zhang
,
Qun Wang
,
Chidambaram Seshadri Ramachandran
,
Peng Guo
,
Aiying Wang

The morphology (surface and cross-section) of the high-velocity oxygen–fuel coupled physical vapor deposition (HVOF-PVD)  duplex coating and its mechanical (stress and load-bearing capacity), tribological (wear and lubrication), and corrosion (chemical and electrochemical) behavior are described and discussed for surface protective concern. The duplex coating involves a combination of a thick, hard HVOF bottom layer and a thin, hard yet tough PVD top layer. The hardness and modulus of the HVOF coating are between those of the soft substrate and the hard PVD coating, which thereafter provides a smooth transition and effective support between them. As a result, the HVOF interlayer can eliminate the plastic deformation of the substrate due to stress concentration under heavy load, effectively avoiding the “eggshell effect”. As expected, the duplex coating system would provide excellent load-bearing capacity. In terms of tribology, the wear resistance of the HVOF-PVD duplex coating is significantly improved compared to that of PVD monolayer coating. 

  • thermal spray
  • HVOF
  • PVD
  • microstructure
  • mechanical properties

1. Introduction

Thermal spraying is a general term for a series of coating processes. The process involves the powder material being given thermal and kinetic energy in a hot air flow medium and sprayed onto the substrate at high speed to form a coating [1][2][3][4]. Over the past few decades, a series of thermal spray technologies have been developed, such as electric arc spraying (EAS), plasma spraying (PS), detonation spraying (DS), and high-speed oxy-fuel (HVOF) spraying [5][6][7][8][9]. In particular, HVOF spraying achieves a favorable combination of thermal and kinetic energy due to its high speed (~1500 m/s) and relatively low temperature (~2000 °C) [10]. In addition, it benefits the advantage of high flexibility (such as the deposition of cermets, oxide and oxide-free ceramics coatings) and low cost for mass production [11][12]. Thus, HVOF coating as a protective candidate has been widely used in various industrial fields, such as aviation, aerospace, petroleum, and marine components as well as systems [13][14][15]. It can be observed that the raw material powder and the spraying process are two key factors for the coatings prepared by HVOF spraying. Therefore, designing and optimizing the raw material powder and spraying process parameters is quite crucial before coating deposition. Typically, the powder material consists of a metal/ceramic primary phase and metal binder phase (Co, Cr, Ni, etc.) with a particle size distribution of 10–40 μm [16][17][18]. At present, a variety of commercial feedstock powders have been developed, mainly including but not limited to WC-based, Cr3C2-based, and Fe-based powders [19][20][21]. Moreover, many researchers have conducted structural and compositional modifications based on commercial feedstock powders to obtain high-quality HVOF spray coatings [22][23][24]. In addition, the HVOF spraying parameters particularly involving the feature and flow rate of working gas and the ratio of oxygen/fuel gas make a great contribution to the temperature, airflow velocity and decomposed composition during deposition, which could change the structure and properties of coatings, such as strength, toughness and residual stresses, to a great extent [10][25]. As a result, it is empirically known that the performance of HVOF spray coating shows strong dependence upon the combined effect of feedstock powder materials and the spraying processes.

2. Mechanical Properties

The mechanical properties of the duplex coating are primarily determined by the PVD top layer before it delaminates/flakes off. Therefore, to prevent premature failure of the brittle PVD coatings under high contact stress states, the stress distribution and load-bearing capacity of duplex coatings are particularly essential. In duplex coating designs, the HVOF coating acts as an intermediate layer between the soft substrate and the brittle PVD coating, which typically lies in between in terms of hardness and modulus. It can provide an effective transition and support for the PVD top layer and soft substrate. As a result, the plastic deformation of the substrate due to stress concentration can be avoided when heavy loads are applied, thereby significantly improving the load-bearing capacity of the duplex coating. The contact stress distribution of the material can be visualized from the von Mises stress contour plot obtained through finite element simulations.
For the PVD coating, the stress concentration region is mainly at the interface of the coating and the substrate. Soft substrates are prone to plastic deformation under high contact stress conditions, which in turn leads to spalling failure of brittle PVD coatings. However, in the duplex coating the stress concentration region is confined to the thick and hard HVOF interlayer. Thus, the substrate only bears weak contact stress. In this case, the PVD top layer will not fail rapidly due to the plastic deformation of the substrate. Li et al. [26] investigated the load-bearing capacity of duplex coatings by Vickers indentation and scratching. They studied the effect of different HVOF (Cr3C2-NiCr) interlayer thicknesses on the load-bearing capacity of duplex coatings. The average indentation diagonal size of the PVD coating is the largest (52.5 μm) when there is no HVOF interlayer. It indicates that soft alloy substrates have difficulty in providing sufficient support for PVD coatings to withstand heavy loads [27]. With the addition of the HVOF interlayer, the load-bearing capacity of the PVD top layer is significantly enhanced.
It can be understood that the HVOF interlayer provides effective support for the PVD top layer. In addition, the scratch test results also show that the HVOF interlayer can significantly improve the interfacial bond strength of the PVD coating. In conclusion, the dual-phase coating design greatly eliminates the stress concentration at the interface between the PVD coating and the soft substrate, effectively inhibiting the plastic deformation of the substrate under heavy load, thereby significantly improving the load-bearing capacity of the PVD coating [28]. Incidentally, once the thickness of the HVOF interlayer covers the stress concentration region, its thickness value has little effect on the load-bearing capacity of the PVD coating. This point can be concluded from the simulation and experimental results of Zheng and Li et al. [26][29].

3. Tribological Performance

Tribological properties consist of antifriction and wear resistance, respectively. The low coefficient of friction and wear rate signifies excellent antifriction and wear resistance [30][31][32]. Initially, the HVOF interlayer was introduced between the PVD top layer and the soft substrate to avoid delamination/collapse of the brittle PVD coating due to the “eggshell effect” under repeated heavy loading. Therefore, it is not difficult to understand that the wear resistance of the HVOF-PVD duplex coating is significantly improved compared to the PVD single layer. However, the surface roughness of the duplex coating is higher than that of the PVD coating [33]. As a result, the frictional resistance is greater during the sliding process, causing an increase in the coefficient of friction. In short, compared with PVD coating, HVOF-PVD duplex coating has superior wear resistance, but the antifriction is somewhat reduced. It can be seen that both CrN-based and DLC-based duplex coatings have comparable coefficients of friction to the respective PVD single layer. That is to say, the presence of the HVOF interlayer seems to be less important for the antifriction properties of PVD coatings. It has been reported that the HVOF interlayer adversely affects the antifriction performance of the PVD top layer [26]. It can be found that the friction coefficient of the PVD monolayer is consistently lower and less fluctuating than that of the duplex coating. This can be explained by the surface quality of the coating. As mentioned in Section 2, the surface roughness of the HVOF-PVD duplex coating is greater than that of the PVD monolayer due to the growth defects during the physical vapor deposition [33][34][35], and the rough surface will inevitably cause greater frictional resistance. Therefore, during the friction process, the friction coefficient of the duplex coating is higher than that of the PVD monolayer.
While under the same test parameters, the PVD coating in the HVOF-PVD duplex coating was not severely delaminated, which only showed deep furrows and small separations under shear action. It is not difficult to find that the wear resistance of the duplex coating has made a qualitative leap after introducing the HVOF interlayer between the soft substrate and the PVD coating. It can be attributed to the successful transfer of surface contact stresses and improved adhesion in the duplex coating system [36]. It should be further pointed out that the hardness and thickness of the HVOF interlayer will also affect the wear resistance of the duplex coating to a certain extent [26][37]. However, the impact of these factors appears to be negligible.

4. Corrosion Behavior

The surface of the HVOF coating is covered with a specific PVD coating, which can enhance its corrosion resistance, including chemical and electrochemical corrosion [38][39][40]. Generally, HVOF coatings present more intrinsic defects, such as pores and cracks, which provide more corrosion channels for corrosive media (Cl-). In contrast, PVD coatings are dense and virtually defect free. Therefore, the PVD top layer in the duplex coating system can effectively inhibit the corrosive medium Cl- from entering the interior to penetrate the passivation film (oxide film), thereby delaying the swelling, flaking, and pitting formation of the passivation film. In turn, the corrosion resistance of the HVOF coating is substantially improved. As is well known, the electrochemical corrosion behavior of coatings can be quantitatively evaluated by the potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) provided by electrochemical workstations [41][42]. Monticelli et al. [40] investigated the electrochemical corrosion behavior of the HVOF coating and HVOF-PVD duplex coating on steel substrates in a 3.5% NaCl solution. It can be visually observed that the corrosion resistance of the coated samples is better than that of the steel substrate. It can be seen that the current densities of both HVOF-PVD duplex coatings are lower than the respective HVOF coatings. This means that the PVD coating coverage further enhances the corrosion resistance of the HVOF coating. The presence of the coating greatly enhances the resistance of the sample to aggressive ions (Cl-) [43]. Therefore, it is easy to understand that the corrosion resistance of the coated samples is superior to that of the steel substrate.
However, HVOF coatings have relatively more pores and microcracks, which provide channels for the penetration/diffusion of corrosive media. It will adversely affect the corrosion resistance of the HVOF coating. In this case, the dense PVD coating on the HVOF coating can serve to seal the pores and block/extend the corrosion channels. Therefore, the PVD coating can be regarded as a strong barrier layer, further preventing the corrosion attack of Cl- on the HVOF coating and even the substrate. It is the major reason for the superior corrosion resistance of HVOF-PVD duplex coating compared to HVOF coating. As mentioned earlier, EIS is also an important tool to evaluate the electrochemical behavior of coatings. It can be seen from the Bode plot that the phase angles of both HVOF-PVD duplex coatings are always above the HVOF coating. Moreover, the impedance modulus value of the duplex coating is also much higher than that of the HVOF coating. It is further observed from the Nyquist plot that the capacitive arc radius of the two HVOF-PVD duplex coatings is significantly larger than that of the HVOF coating. These results indicate that the HVOF-PVD duplex coating plays a more prominent role in hindering the penetration of the corrosion solution than the HVOF coating [44][45]. It is worth mentioning that DLC-covered HVOF coating has a phase angle closer to 90°, a higher impedance modulus, and a larger capacitive arc resistance radius compared to CrN. This indicates that the chemically inert DLC can improve the corrosion resistance of HVOF coatings more significantly than CrN [46]. In short, the corrosion resistance of HVOF-PVD duplex coating is distinctly better than that of HVOF coating, and it is closely related to the type of PVD coating.
In addition to electrochemical corrosion, chemical corrosion is also another common corrosion phenomenon that produces loss and damage to materials [47][48]. Tang et al. [39] studied the hot corrosion behavior of HVOF (Cr3C2-NiCr) coatings and HVOF-PVD (Cr3C2-NiCr/CrN) duplex coatings in mixed salts at 550 °C.
It can be seen that the oxide layer of the HVOF coating exhibits a dark-scale structure with a large number of voids inside. Moreover, the thickness of the oxide layer reaches 13 μm (elemental distribution of oxygen in EDX scan results). In contrast, the oxidation erosion of HVOF-PVD duplex coatings is very restricted. The PVD coating remains relatively intact after hot corrosion, and the thin oxide layer (~1.3 μm) still maintains a dense structure. However, due to the growth defect of the PVD coating [35], localized corrosion occurred in the regions that were not completely covered by the HVOF coating. Based on the above phenomena, it can be known that the coverage of the PVD coating dramatically improves the chemical corrosion resistance of the HVOF coating. This is due to the high number of defects in the HVOF coating, which allows the corrosive medium (Cl−) to easily enter the interior of the coating. The extremely penetrating Cl− breaks through the oxide film and triggers oxidation expansion of the inner layer, which in turn causes cracking and flaking [39][49][50][51]. For comparison, since the duplex coating has a very dense PVD top layer, it is difficult for the corrosive medium to enter the interior of the coating to attack and destroy. Therefore, under the protection of PVD coating, the HVOF-PVD duplex coating exhibits outstanding hot corrosion resistance.

This entry is adapted from the peer-reviewed paper 10.3390/coatings12101395

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