**4. Material system and structure design of SPE coating**

### **4.1 Material system of erosion resistant coating**

In the past 25 years, there have been many studies on anti-erosion coating material systems, most of which are comparative studies aimed at evaluating the performance of specific materials. Most studies focus on two coating series: one is carbon based system [2, 18, 19] (diamond, diamond-like carbon (DLC), tetrahedral amorphous carbon (ta-C) and some carbides); The second is a system based on nitrides (mainly TiN) [20–26] (TiN, TiAlN, TiCN, TiSiN, TiSiCN, etc.). Other nitrides (CrN [27, 28], ZrN [21], etc.) have also been partially studied.

Diamond coating has many excellent properties, most notably high hardness and strength, which makes it an attractive choice for friction and wear resistant parts. Therefore, many researchers have carried out detailed experimental studies on the erosion resistance of diamond coatings, and have a more in-depth understanding and analysis of the impact of erosion grit material, shape, size, erosion speed and angle on the erosion resistance and erosion mechanism.

Wheeler et al. [2] have studied the erosion resistance of diamond coatings in detail. They used Chemical vapor deposition technology to prepare diamond coatings of 10– 46 μm thickness, and carried out erosion tests at 90° angles of attack using grit with different average diameters and velocities, the erosion resistance was studied. The erosion rate is related to the kinetic energy of the erosion particles, and compared with tungsten carbide and stainless steel. It is concluded that the erosion mechanism of diamond coating is composed of three stages. First, the coating produces microcracks; Secondly, pinholes and interfacial debonding occur; The last is the complete failure of the coating. Wheeler and others also studied the impact of different impact angles on the erosion performance of diamond coatings. The results show that although the number of impacts required at the beginning of pinhole increases significantly at small angles, all angles will produce a "pinhole" damage feature. As shown in **Figure 5**, circular cracks and pinholes are observed on the eroded surface, and can be observed on the eroded coating at all angles. As a part of establishing their formation mechanism, the author measured the diameter of annular cracks, analyzed the Hertz fracture theory and proposed the stress wave reflection theory. According to these results, the author also proposed to use energy to describe the impact of solid particles on the erosion resistance

#### **Figure 5.**

*Micrographs of the surrounding cracks and pinholes of diamond coating under multi-angle impact [2] (a) 30°, (b) 60°, and (c) 90°.*

of the coating: *E*EP = *E*<sup>c</sup> + *E*E, where *E*EP is considered to be the total energy consumed by the elastoplastic damage caused by a single impact, while *E*<sup>c</sup> and *E*<sup>E</sup> are the energy dissipated by the coating and erosion damage, respectively.

Till now, TiN based coatings have been the most widely used SPE resistant coatings in aeroengines. Therefore, many researchers still focus on TiN and its multiple systems. In the past 20 years, more and more researchers have begun to study nanocomposite TiN based coatings with better mechanical properties. The research results of Reed et al. [29] show that the erosion resistance of (Ti, Cr) N nano coating is highly dependent on the coating thickness and Cr content, and the increase of CrN phase volume reduces the hardness of the coating. Under the attack angle of 30°, all the coated samples are better than the uncoated substrate; However, under the condition of 90° attack angle, the coating deposited at low bias voltage (25 V, 50 V, and 100 V) and Ti: Cr ratio (N = 2.4) is superior to the uncoated substrate. Therefore, hardness does not play a decisive role in the erosion resistance of the coating.

In recent years, CrN coating has attracted extensive attention of researchers because of its excellent toughness, wear resistance, high temperature oxidation resistance and corrosion resistance, and low residual stress of the coating, which makes it easy to deposit thicker coatings [30, 31]. However, the hardness of CrN coating is low, only about 1800–2000 Hv. At present, multiple nitrides [32–34] can be formed by doping elements (such as Al, Ti, TiAl, Si, etc.) to improve the hardness of the coating. Among them, Al and N in CrAlN coating are bound by covalent bond, and the grains of the coating are uniform and fine, which not only improves the hardness to about 2500–3000 Hv, but also increases the thermal stability of the coating. In the high temperature environment, Al atoms and Cr atoms are easy to diffuse outward, and combine with oxygen to form a more compact Cr2O3 and Al2O3 oxide layer. After the formation of this oxide layer, the volume expands, forming a compressive stress on the coating, which can resist the crack initiation on the surface, effectively prevent the deep level of oxidation, and improve the high-temperature oxidation resistance of the coating [35, 36]. According to the experience in preparing coatings in the field of engineering machinery, the most common method to improve the mechanical properties of metal nitrides, including hardness, toughness and oxidation resistance, is to add Al. Many studies show that the nitride coating containing Al still shows good wear resistance at high temperature [37–39]. Ren et al. [40] deposited CrAlN and CrN coatings on the steel substrate respectively. They found that the addition of Al makes some Cr atoms replaced by Al atoms to form CrAlN phase, which is conducive to refining grains and improving the comprehensive performance of CrAlN coatings. Wu et al. [41] deposited four kinds of coatings: TiN, TiAlN, CrAlN and CrAlTiN on the steel substrate. It is found that the CrAlN coating has the best erosion resistance, which is 3.9 times of the substrate.

According to the erosion model proposed by Evans et al. [42] and improved by Hockey et al. [43], they believe that the volume loss of brittle materials during erosion is proportional to the velocity, radius, density and impact angle of erosion particles, and inversely proportional to the fracture toughness and hardness of coating materials. Under the condition of high angle of attack, micro brittle fracture caused by elastic-plastic deformation is one of the main modes of erosion damage of hard coatings. According to the micro brittle fracture theory [42], the erosion volume (W) of materials can be expressed as:

$$\mathbf{W} = \mathbf{C} \mathbf{K}^{-\frac{4}{3}} \mathbf{H}^{-\frac{1}{4}} \tag{1}$$

Where: *W* – Erosion volume; *H* – hardness; *K* – toughness; *C* – Constant (depending on particle size, velocity and particle density of erosive particles). It can be seen from Eq. (1) that the erosion resistance of the coating can be improved by improving the hardness and toughness of the coating material. Moreover, improving the toughness has a more significant effect on improving the erosion resistance of the coating. Therefore, when selecting coating materials, it is necessary to comprehensively consider the hardness and toughness. On the basis of ensuring a certain hardness, it is necessary to focus on improving the toughness. It should be emphasized that it is meaningless to simply discuss toughness instead of strength (or hardness). Only those hard and tough coating materials have engineering application value.

#### **4.2 Structural design of erosion resistant coating**

With the deepening of research, some defects of monolayer hard coating in the field of erosion resistance have been revealed: the high stress of the coating, the high brittleness, and the low toughness, which leads to cracks easily appear when the coating is eroded by solid particles [44]. According to the literature, the maximum thickness of metal nitride coating with monolayer structure is about 6–8 μm [45]. However, when the coating thickness is thin, its erosion resistance is difficult to meet the protection requirements of related parts. At present, gradient multilayer coatings with complex structure have become the research focus of erosion resistant coatings. This kind of coating has a large number of interfaces, through which the continuous growth of columnar crystals can be restrained, the energy of erosion particles can be dissipated, and the initiation of crack sources and the propagation of buffer cracks can be prevented; At the same time, the interlayer can release the residual stress to a certain extent, coordinate the deformation, and improve the film substrate bonding strength and coating toughness.

According to the principle of multilayer strengthening, the difference of crystal structure and elastic modulus is the main reason for the dislocation to be blocked at the interface, thus forming the strengthening. In the multi-layer structure with alternating soft and hard, even if the metal (Me) in the metal layer and ceramic layer is the same element, the crystal structures of Me and MeN are different, so they will not grow epitaxial structures. The difference of elastic modulus will form a certain strengthening effect. Zhang et al. [46] studied the evolution of the cyclic impact damage mechanism of TiN/Ti multilayers with the sharp shortening of the modulation period from micrometer (1000 nm) to nanometer (60 nm). The results show that with the decrease of modulation period, the ductile phase of the films decreases, and the microstructure changes from TiN/TixNy/Ti to TiN/TixNy (x > y). The results of cyclic impact show that the impact resistance and damage mechanism of TiN/Ti

multilayers are closely related to the modulation period. The smaller the modulation period of TiN/Ti multilayers, the lower the critical fracture load, the higher the fracture probability, and the worse the impact resistance.

Wieciński et al. [47] studied the erosion resistance and fracture mechanism of nanostructured Cr/CrN multilayer coatings. The researcher deposited seven multilayer coatings with different modulation ratios (Cr/CrN) by arc ion plating. These coatings have the same thickness (5–6 μm) and 16 layers of Cr/CrN, but the thickness ratio (QCr/CrN) of Cr and CrN component layers is different. The small particles of silicon dioxide used in the erosion experiment impact the coating surface at an angle of attack of 90°. **Figure 6** shows the morphology of Cr/CrN multilayer coating before and after erosion. It can be seen that after erosion, the Cr layer grains uniformly elongate along the interface and rotate by 90° compared with that before erosion. This change may be due to dislocation sliding between grains. The microstructure change of Cr layer is caused by plastic strain produced by erosion particles, while the microstructure (grain size and shape) of CrN layer is basically unchanged. Compared with the columnar structure, the microstructure composed of Cr/CrN multilayer grains grown at different interfaces can effectively prevent the propagation and propagation of cracks along the grain boundaries.

Therefore, the essence of multilayer structure to improve coating toughness is deeply explored [48]. Its toughening mechanism usually comes from the following aspects: (1) the interlayer interface deflects cracks; (2) The soft phase layer has better plastic deformation ability and can relieve the interface stress at the same time; (3) The crack tip is wrapped by a soft phase layer, which can passivate the crack tip, inhibit crack growth, and improve the toughness of the coating material (**Figure 7**). In simple terms, in multilayer coating structures, the metal phase is used to absorb

**Figure 6.** *Microstructure of Cr/CrN multilayer coating (a) before erosion test, and (b) after erosion test [47].*

**Figure 7.**

*Schematic diagram of the toughening mechanism of multilayer film structure [48].*

excessive plastic deformation, while the ceramic phase provides hardness and wear resistance. It can be seen that the multilayer structure improves the toughness of the coating by a variety of mechanisms. These toughening mechanisms have been verified in a number of studies.

In general, the ductile interlayer improves the erosion resistance of the hard coating by improving the film substrate bonding strength and toughness. In the structural design of erosion resistant coatings, the multilayer structure of ductile interlayer/hard surface layer can be used for reference as the research direction of strengthening and toughening coatings.
