**1. Introduction**

The present scenario of the automotive engineering is concerned with the higher power generation. There is a huge requirement of the materials which can sustain the high temperature in order to increase the power generation of the systems. These materials require high thermo-mechanical properties to withstand the hostile conditions like oxidation and erosion at elevated temperatures up to 900°C. The surface of the components deteriorates rapidly at such aggressive conditions. Grey cast iron (GCI) is one of the most commonly used materials in the turbocharger housing and its integral parts such as housing of bearing. The GCI components must work satisfactorily for prolonged duration of time under such aggressive working condition [1–4]. However, the GCI alone cannot provide enough oxidation and erosion

resistance at high temperature. Thus, there is a need to modify the surface properties of the GCI component which enables it to work under high temperature oxidation and erosion environment. There are number of methods to modify the surface properties of materials such as by changing the surface chemistry of the materials with the help of surface modification techniques such as carburizing, nitriding and cyaniding, etc. The various methods of surface engineering applied for surface modification are given in **Table 1**. The methods employing the deposition of a layer of superior material is commonly employed in surface engineering. This can be


## **Table 1.**

*Surface engineering options and property enhancement.*

### *High Temperature Oxidation and Wear Resistant Bi-Layer Coating for Turbocharger Housing DOI: http://dx.doi.org/10.5772/intechopen.86806*

done by number of methods such as weld overlaying, cladding and thermal spraying, etc. Nowadays, the thermal spraying methods are used in various industries due to their specific advantages such as ease of operation, enhanced surface properties, less detrimental effects to the substrate and to produce the coatings on any intricate shaped components [5–7]. This process is utilized in the deposition of almost all class of engineering materials like polymers, ceramic, metal, and composite [8, 9].

There are various techniques of thermal spraying but the high velocity oxy-fuel (HVOF) process is gaining popularity due to its specific characteristics such as corrosion and wear resistant coating, high velocity up to 1000 m/s, lower porosity content and dimensional restoration. This high velocity in this technique causes feedstock powder to deform plastically on the surface of the substrate [10]. Moreover, this technique has taken up by numerous industries due to its mobility, economical processing and ease of operation with safety along with excellent quality of the coating. High velocity oxy-fuel (HVOF) process provides the coatings with high hardness, very low oxidation, good abrasion resistance, lower porosity levels and high erosion resistance as compared to the other thermal spray coating techniques [11]. The characteristics of high velocity makes it enable to deposit coatings with improved bonding with the substrate that results in higher hardness, less porosity and high density. The various attributes of different thermal spray methods involves the type of heat source used, which is responsible for the temperature of the process, the velocity of powder particles and the rate at which powder is being feed for the deposition purpose. The comparison of various characteristics of thermal spray coating processes is given in **Table 2**.

Oxidation is related to the formation of some undesired phases like porous and non-coherent oxides on the surface of the material, which is responsible for the failure of the component. The oxidation generally starts from the surface due to diffusion of oxygen and further propagates to the sub-surface of the material in an aggressive thermal cycling condition. This problem is observed in turbocharger housing and bearing housing. These components also suffer from an erosive wear along with the oxidation. In the erosion of components, the hot air strikes on the surface of the components in cyclic and repetitive manner, and this air contains some unwanted dust particles. The intensity of these particles and hostile environment are two major reasons responsible for the degradation of the surface with erosion at elevated temperature above 500°C [12]. The oxidation of component is not only a single problem to solved by using Ni-based coatings, the erosion is also present due to the erodent particles impacting on materials surface at high velocities; thus there is a requirement of feasible solution which can help to minimise both the problems of oxidation and erosion at a same time.


#### **Table 2.**

*Attributes for different thermal spray processes.*

Nowadays, the Ni-Cr coatings are used as a most preferred candidate for the oxidation resistance of components operated at high temperature range up to 650°C. However, these coatings suffered a problem of diffusion of Fe element from the substrate to the coating, when applied on the ferrous materials against oxidation at high temperature. The Ni-50Cr coatings were also used to minimize this problem by using HVOF process [13]. The composite coatings then came into use and were utilized to provide resistance against erosion and corrosion at high temperature. The Cr3C2-NiCr, NiAl-40Al2O3, and Cr3C2/TiC-NiCrMo composite coatings were deposited on low carbon steel to check its performance against erosion-corrosion at 400°C. The inter-metallic present in NiAl-40Al2O3 was responsible for the maximum resistance to the erosion-corrosion [14].

In the recent time, the superalloy materials are extensively used for high temperature applications. Their versatile use in these applications is due to fact that they can maintain their high strength, low temperature ductility and excellent thermal stability. Superalloys are mainly precipitation hardened or solid solution hardened nickel, iron, and cobalt based heat resistant alloys, which exhibit various properties such as good mechanical strength, wear resistance and high temperature stability. These superalloys are further classified into cast, wrought and powder metallurgy alloys. A general classification of superalloys is shown in **Figure 1**. More than 50% of the primary materials used in the hot portions of the gas turbine engines such as blades, vanes and combustion chamber are superalloys. Typical applications of superalloys include gas turbines and jet engines [15]. The versatile use of superalloys in such applications is due to fact that the thermodynamic efficiency of turbine engines is increased with increased turbine inlet temperature. These motivated the designers for increasing the maximum operating temperature using the superalloys. The Ni-based superalloys are most commonly used in high temperature applications, followed by the cobalt-based alloys and then the iron-nickel alloys. The Ni-based alloys are preferred for high temperature applications because of their excellent oxidation resistance at high temperature. The Ni-based superalloys are most stable materials at high temperature due to their face-centered cubic (FCC) crystal structure. FCC crystal structure is required at high temperature because

*High Temperature Oxidation and Wear Resistant Bi-Layer Coating for Turbocharger Housing DOI: http://dx.doi.org/10.5772/intechopen.86806*

of its close packed structure. Consequently, the creep, which is more prominent at high temperature, is less in FCC crystal structure. Thus, in the current research work, a nickel based superalloy Alloy-718 was selected as a feedstock powder for increasing performance of GCI at high temperature application. Alloy-718 is well known for its thermo-mechanical strength at elevated temperatures [16]. The NiCrAlY was also deposited before the coating of Alloy-718 as a bond coat material. The NiCrAlY also provides a rough surface for Alloy-718 top coat and also restricts the diffusion of elements from substrate to coating and vice-versa [17]. Hence, a bi-layer of Alloy-718/NiCrAlY was deposited on the GCI substrate by using thermal coating technique known as high velocity oxy-fuel (HVOF) spray process. The deposited bi-layer coating was investigated for its performance against high temperature erosion and oxidation. The oxidation test was conducted for 50 cycles at 900°C in a tube furnace, whereas the erosion test was conducted by using high temperature air-jet erosion testing rig at 800°C. The microstructure and mechanical characterization were performed to analyse the properties and performance of deposited bi-layer coating. The mechanical characterization mainly included the micro-hardness testing and erosion test, whereas, the microstructural analysis covers the porosity analysis, inter-splat bonding and morphology of the sprayed coating.
