**5. Research and application status of SPE coating**

In the past 30 years, the United States, Russia and other countries have cooperated with GE, Canada MDS, Liburdi and Russia PRAD, etc., and carried out research work in terms of material system, preparation technology and process, engineering test assessment and evaluation. Now, this technology has been successfully applied to dozens of models of engines.

As early as 1988, Liburdi Company in Canada began to develop erosion resistant coatings, and the coatings were gradually applied to various types of engines until the early twenty-first century. From 2000 to 2003, the anti-erosion coating prepared by the company was applied to T55, T58, T64 and AE1107 engines. In 2004, the company processed more than 2000 sets of T56 compressor blades with erosion resistant coatings for Rolls Royce. Since 2007, the Saudi and Jordanian Air Forces selected the T56 engine (compressor blades are provided with erosion resistant coatings prepared by Liburdi). In 2008, the third generation anti erosion coating (mainly TiAlN, as shown in **Figure 8**) was introduced, which can make the service life of the engine more than three times that of the uncoated engine, and improve the engine performance by 3% [44].

On the basis of the extensive application of binary anti-erosion coating, MDS-PRAD and GE further improved the material and structure of the anti-erosion coating, and applied ER-7 and Black Gold ceramic coatings, which are mainly composed of TiN and TiAlN, to helicopter and transport engine blades. The main component of ER�7 coating is TiN. It adopts a multilayer structure with alternating soft and hard. The substrate is a hard and dense nickel base metal. A transition layer between the substrate and the coating is used to improve the film substrate bonding strength. At the same time, the coating has strong resistance to fatigue crack growth and multi angle sand erosion.

## *Solid Particle Erosion DOI: http://dx.doi.org/10.5772/intechopen.109383*

Praxair Surface Technologies has developed a sub stoichiometric TiN/TiN1�<sup>x</sup> (called "24k Type II™") Compared with the traditional TiN coating, its erosion resistance has been greatly improved. This coating system has been tested to provide excellent gravel erosion protection in a variety of aircraft engines, including civil engines. Due to the increased demand for erosion resistant coatings in desert environments, the U.S. Navy has implemented a plan to extend the life of compressor blades of T64 helicopter engines by applying erosion resistant coatings [49, 50]. The company has established a production base, which can effectively prepare 24 k Type II™ Multilayer coating, and has become one of the major suppliers in aviation applications. Since then, the coating has been applied to hundreds of thousands of blades. The coating structure is shown in **Figure 9** TiN1�<sup>x</sup> layer thickness is less than 0.2 μm, TiN layer thickness is 1 μm. The total coating thickness is usually 15–25 μm. **Figure 10** shows the comparison diagram of erosion rate of multilayer coating and monolayer TiN coating, thermal spraying coating (chromium carbide and tungsten carbide) and Ti-6Al-4 V base material at 20° and 90° attack angles. The results show that the multilayer coating is obviously superior to other coatings [51].

Oerlikon Balzers has also provided BALINIT TURBINE PRO coating for erosion protection in recent years (the relevant performance is shown in **Figure 11**). This coating uses TiAlN series multilayer structure to achieve the best matching of high hardness and residual compressive stress, providing excellent erosion protection performance.

**Figure 9.** *TiN/TiN1*�*<sup>x</sup> "24k Type II™" multilayer coating structure [51].*

#### **Figure 10.**

*Erosion rates of Ti-6Al-4 V base material, thermal sprayed Cr-C and WC coating,TiN coating, and TiN/TiN1*�*<sup>x</sup> "24k Type II™" multilayer coating at attack angles of 20° and 90°, respectively [51].*


## **Figure 11.**

#### *Basic properties of BALINIT TURBINE PRO coating.*

German MTU Company has designed and developed erosion resistant coatings (ERCoatnt Generation I and II) for aircraft engine compressor blades, as shown in **Figure 12**. **Figure 13a** shows the metallographic micro-section of the coating, clearly showing the total thickness of 25 μm and the ceramic and metal interlayer of about 3 μm per cycle. **Figure 13b** shows the high-resolution scanning electron micrograph of the coating, showing that the ceramic interlayer is a nano multilayer structure. The chemical composition of the continuous nano layers varies slightly, and the thickness of each nano layer is only 20 nm to 50 nm. Compared with traditional coatings, this nano design again significantly reduces the size of potential cracks or defects. In general, it is essentially chemical composition, and nano design and multilayer structure jointly achieve the ideal erosion resistance of the coating (**Figure 14**). **Figure 15** shows the effect of ERCoatnt coating on high cycle fatigue and low cycle fatigue

**Figure 12.** *First-generation coating (TiN series) second-generation coating (TiAlN series) [40].*

#### **Figure 13.**

*Multilayer structure of ERCoatnt coating [40]: (a) cross section (b) high resolution electron microscope cross section morphology.*

**Figure 14.** *Erosion test results of coated and uncoated titanium samples [40].*

#### **Figure 15.**

*The effect of erosion-resistant coating on the fatigue strength of titanium alloy high-cycle (HCF) and low-cycle (LCF) (a) influence on HCF intensity (b) influence on LCF intensity [40].*

strength of titanium alloy. The test data showed that the high cycle fatigue strength decreased by as much as 15% depending on the coating system, the substrate material and the selected geometry. When the critical strain level is exceeded, ERCoatnt coating may accelerate crack initiation under low cycle fatigue load [40].

The team of Guangdong Academy of Sciences Institute of New Materials successfully developed 5–30 μm thick TiN based and CrN based alternating soft and hard multilayers by using vacuum cathodic arc ion plating technology [52–57]. The results show that: (1) The multilayers have good comprehensive properties: the thickness can reach 20 μm or more (see **Figure 16**), the adhesion is greater than 70 N, and the

**Figure 16.** *SEM cross-sectional morphology of Ti-TiN-Zr-ZrN multilayer film [52].*

hardness is greater than 30 GPa. It has a good anti-erosion protection effect on titanium alloy and steel base materials. (2) The coating prepared by arc has large metal particles which are not ionized, and these particles lead to a significant reduction in the overall performance of the coating, especially the erosion resistance. And (3) Due to the high hardness, the existing hard coatings form an "eggshell effect" with the titanium alloy substrate, which has a negative impact on the high stress low cycle fatigue performance of the substrate.
